Subsequent Event - Subsequent Event (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Jan. 31, 2018
Subsequent Event [Line Items]
Repurchase of common stock	$ 184,000,000
Remaining authorized repurchase of common stock, amount	$ 316,000,000
Subsequent Event
Subsequent Event [Line Items]
Stock repurchase program, increase (decrease), authorized amount		$ 250,000,000
Stock repurchase program, authorized amount		$ 500,000,000