Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Jan. 21, 2015
Share repurchase program
Stock Repurchase Program, Number of Shares Authorized to be Repurchased		20,000,000.0
UpfrontPaymentUnderAcceleratedStockRepurchaseProgram	$ 50.0
Stock Repurchased and Retired During Period, Shares	3,100,000
Stock Repurchased and Retired During Period, Value	$ 40.0
UnsettledForwardContractIndexedtoIssuersStockClassifiedwithinStock	$ 10.0
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	4,300,000
Retained Earnings [Member]
Share repurchase program
Stock Repurchased and Retired During Period, Value	$ 29.7