Shareholders' Equity - Shares Repurchased (Details) - USD ($)	2 Months Ended	12 Months Ended
	Feb. 23, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 01, 2017	Jan. 06, 2017
Equity, Class of Treasury Stock [Line Items]
Total payments		$ 501,000,000	$ 306,000,000	$ 555,000,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Authorized repurchase amount					$ 300,000,000
Number of Shares Repurchased (in shares)		12,669,643	10,721,248	20,995,419
Total payments		$ 501,000,000	$ 306,000,000	$ 555,000,000
Average price paid (in dollars per share)		$ 39.57	$ 28.53	$ 26.43
Subsequent Event [Member] | Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	$ 305,000,000
Number of Shares Repurchased (in shares)	1,230,941
Total payments	$ 43,000,000
Average price paid (in dollars per share)	$ 34.90
Chief Executive Officer [Member]
Equity, Class of Treasury Stock [Line Items]
Deferred equity compensation (in shares)						297,131