Stockholder's Equity (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stockholder's Equity
Taxes paid related to net share settlement on equity awards	$ 1,520,000	$ 806,000	$ 258,000
Allocated Share-based Compensation Expense	4,200,000	2,700,000	1,900,000
Expected volatility rate (as a percent)	25.60%
Risk free interest rate (as a percent)	0.35%
Dividend rate (as a percent)	0.00%
Incremental value treated as additional expense	600,000
Restricted Stock Units
Stockholder's Equity
Unrecognized share-based compensation expense	3,200,000
Expected weighted average recognition period	2 years
Fair value of restricted stock units vested	3,900,000	2,500,000	1,800,000
Shares withheld to cover withholding tax of unit holders	53,775	34,730	15,360
Taxes paid related to net share settlement on equity awards	$ 1,500,000	$ 800,000	$ 300,000