Subsequent Events Subsequent Events (Details Textual) - USD ($)				12 Months Ended
	Feb. 28, 2018	Feb. 20, 2018	Feb. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 14, 2018	Nov. 15, 2017	Oct. 17, 2016
Subsequent Event [Line Items]
Stock Repurchase Program, Authorized Amount								$ 150,000,000	$ 400,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount				$ 112,500,000
Dividends declared per share (usd per share)				$ 2.00	$ 2	$ 2.00
Subsequent Event [Member]
Subsequent Event [Line Items]
Stock Repurchase Program, Authorized Amount							$ 350,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount							$ 350,000,000
Dividends Payable, Date to be Paid	Feb. 28, 2018
Dividends Payable, Date of Record		Feb. 20, 2018
Dividend Declared [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Dividends declared per share (usd per share)			$ 0.50