Income Taxes (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Taxes
Effective income tax rate (as a percent)	41.70%	41.60%
Excess tax benefit from exercise of stock options	$ 1,448,000
Tax deductions arising from stock-based compensation	3,900,000
Net unrecognized tax benefits	1,400,000	1,500,000
Accrued liabilities for interest expense and penalties related to unrecognized tax benefits	$ 300,000	$ 200,000