Shareholders' Equity - Narrative (Details) - USD ($)	1 Months Ended					3 Months Ended						6 Months Ended	12 Months Ended
	Jul. 31, 2018	May 31, 2018	Apr. 30, 2018	Jul. 31, 2017	Apr. 30, 2017	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Dividend declared (in dollars per share)						$ 0.60	$ 0.60	$ 0.60	$ 0.48	$ 0.48	$ 0.48
Length of repurchase program		24 months			12 months
Common stock, dividends, per share, cash paid (in dollars per share)			$ 0.60	$ 0.48	$ 0.48		$ 0.60			$ 0.48
Common stock repurchase program, authorized amount		$ 1,000,000,000.0			$ 500,000,000
Treasury stock, shares, acquired (in shares)						1,300,000.0	2,100,000.0						1,800,000.0
Treasury stock, value, acquired, cost method						$ 137,500,000	$ 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
Treasury stock, shares, acquired (in shares)	1,500,000.0
Treasury stock, value, acquired, cost method	$ 162,500,000