Stockholders' Equity (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 14, 2018	Nov. 15, 2017	Oct. 17, 2016
Treasury Stock, Shares, Acquired	188,870	141,358	180,822
Treasury Stock Acquired, Average Cost Per Share	$ 50.17	$ 37.59	$ 46.4645243389
Stock Repurchase Program, Authorized Amount					$ 150,000,000	$ 400,000,000
Stock Repurchased and Retired During Period, Shares	2,081,227	652,835
Stock Repurchased and Retired During Period, Value	$ 99,700,000	$ 27,900,000
Stock Repurchase and Retired During Period, Weighted Average Price	$ 47.89	$ 42.78
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 112,500,000
Subsequent Event [Member]
Stock Repurchase Program, Authorized Amount				$ 350,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount				$ 350,000,000