Shareholders' Equity - Narrative (Details) - USD ($)	1 Months Ended		3 Months Ended				12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2017
Subsequent Event [Line Items]
Dividend declared (in dollars per share)			$ 0.60	$ 0.60	$ 0.48	$ 0.48
Common stock, dividends, per share, cash paid (in dollars per share)		$ 0.48	$ 0.60		$ 0.48
Common stock repurchase program, authorized amount		$ 500,000,000
Treasury stock, shares, acquired (in shares)			2,100,000.0				1,800,000.0
Treasury stock, value, acquired, cost method			$ 275,000,000		$ 500,000,000		$ 225,000,000
Subsequent Event
Subsequent Event [Line Items]
Common stock, dividends, per share, cash paid (in dollars per share)	$ 0.60