(All for US.)
Yes you (will) have a realized long-term capital gain, which is taxable. Long-term gains (including those distributed by a mutual fund or other RIC, and also 'qualified' dividends, both not relevant here) are taxed at lower rates than 'ordinary' income but are still bracketed almost (not quite) like ordinary income, not always 15%.
Specifically if your ordinary taxable income (after deductions and exemptions, equivalent to line 43 minus LTCG/QD) 'ends' in the 25% to 33% brackets, your LTCG/QD income is taxed at 15% unless the total of ordinary+preferred reaches the top of those brackets, then any remainder at 20%. These brackets depend on your filing status and are adjusted yearly for inflation, for 2016 they are:
* single 37,650 to 413,350
* married-joint or widow(er) 75,300 to 413,350
* head-of-household 50,400 to 441,000 (special)
* married-separate 37,650 to 206,675
which I'd guess covers at least the middle three quintiles of the earning/taxpaying population.
OTOH if your ordinary income ends below the 25% bracket, your LTCG/QD income that 'fits' in the lower bracket(s) is taxed at 0% (not at all) and only the portion that would be in the ordinary 25%-and-up brackets is taxed at 15%. IF your ordinary taxable income this year was below those brackets, or you expect next year it will be (possibly due to status/exemption/deduction changes as well as income change), then if all else is equal you are better off realizing the stock gain in the year(s) where some (or more) of it fits in the 0% bracket. If you're over about $400k a similar calculation applies, but you can afford more reliable advice than potential dogs on the Internet.
(update) Near dupe found: see also How are long-term capital gains taxed if the gain pushes income into a new tax bracket?
Also, a warning on estimated payments: in general you are required to pay most of your income tax liability during the year (not wait until April 15); if you underpay by more than 10% or $1000 (whichever is larger) you usually owe a penalty, computed on Form 2210 whose name(?) is frequently and roundly cursed. For most people, whose income is (mostly) from a job, this is handled by payroll withholding which normally comes out close enough to your liability. If you have other income, like investments (as here) or self-employment or pension/retirement/disability/etc, you are supposed to either make estimated payments each 'quarter' (the IRS' quarters are shifted slightly from everyone else's), or increase your withholding, or a combination.
For a large income 'lump' in December that wasn't planned in advance, it won't be practical to adjust withholding. However, if this is the only year increased, there is a safe harbor: if your withholding this year (2016) is enough to pay last year's tax (2015) -- which for most people it is, unless you got a pay cut this year, or a (filed) status change like marrying or having a child -- you get until next April 15 (or next business day -- in 2017 it is actually April 18) to pay the additional amount of this year's tax (2016) without underpayment penalty. However, if you split the gain so that both 2016 and 2017 have income and (thus) taxes higher than normal for you, you will need to make estimated payment(s) and/or increase withholding for 2017.
PS: congratulations on your gain -- and on the patience to hold anything for 10 years!