Capital gains tax rate?

I sold stock this past year, and had capital gains of $183,740.  This sale was considered long-term, and when I put the number into Turbo Tax the effective tax owed came out to $34,544, which I calculated at 18.8%.  Based on my income and the capital gains I profited from the sale, I assumed I would pay a 15% capital gains tax.  Is that not the case?