Stockholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Dec. 31, 2017	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2015	Jan. 21, 2015
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value		$ (40,000)		$ (36,557)	$ (80,000)
Number of shares authorized to be repurchased under the plan						20,000,000.0
UpfrontPaymentUnderAcceleratedStockRepurchaseProgram		$ 50,000			$ 100,000
Stock Repurchased and Retired During Period, Shares	(830,000)	(3,187,000)	(736,000)		(7,812,000)
UnsettledForwardContractIndexedtoIssuersStockClassifiedwithinStock		$ 10,000			$ 20,000
Remaining shares authorized to be repurchased	7,400,000			7,400,000