Capital Stock - Narrative (Details) - USD ($) $ in Millions	1 Months Ended		6 Months Ended
	Jul. 31, 2018	Feb. 28, 2018	Jun. 30, 2018	Dec. 31, 2017
Equity [Abstract]
Treasury stock			34,909,705	14,935,926
Remaining shares available for repurchase (in shares)			10,100,000
Subsequent Event [Line Items]
Incremental shares authorized		25,000,000
Shares authorized for repurchase		30,100,000
Shares repurchased			20,000,000
Treasury stock repurchases (in usd)			$ 481.5
Subsequent Event
Subsequent Event [Line Items]
Incremental shares authorized	20,000,000
Shares authorized for repurchase	30,100,000
Shares repurchased	2,600,000