Stockholders' Equity (Details) - USD ($)			3 Months Ended		6 Months Ended
	May 09, 2017	Feb. 23, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Feb. 28, 2017
Stockholders' Equity
Dividend declared (in dollars per share)	$ 0.48	$ 0.48	$ 0.48	$ 0.48	$ 0.96	$ 0.96
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