Stockholders' Equity (Details) - USD ($)				3 Months Ended		9 Months Ended
	Aug. 09, 2017	May 09, 2017	Feb. 23, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Feb. 28, 2017
Stockholders' Equity
Dividend declared (in dollars per share)	$ 0.48	$ 0.48	$ 0.48	$ 0.48	$ 0.48	$ 1.44	$ 1.44
Shares issued under ATM Agreement						0	0
Authorized amount of share repurchases								$ 500,000,000
Common stock repurchased (in shares)						0	1,052,889
Cost of common stock repurchased							$ 19,723,000
Remaining capacity under repurchase program				$ 262,200,000		$ 262,200,000
Convertible Senior Notes
Stockholders' Equity
Face amount of debt repurchased					$ 0		0
Debt repurchased amount					$ 0		$ 0