Shareholders' Equity - Shares Repurchased (Details) - USD ($)	1 Months Ended	3 Months Ended							9 Months Ended		10 Months Ended	12 Months Ended	70 Months Ended
	Nov. 09, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Nov. 09, 2018	Dec. 31, 2017	Nov. 09, 2018	Aug. 01, 2018	Jan. 06, 2017
Equity, Class of Treasury Stock [Line Items]
Repurchases of common stock									$ 380,000,000	$ 431,000,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased (in shares)		3,299,049	4,163,190	2,787,936	1,934,990	1,847,901	3,456,711	5,430,041				12,669,643
Repurchases of common stock		$ 130,000,000	$ 152,000,000	$ 98,000,000	$ 70,000,000	$ 80,000,000	$ 135,000,000	$ 216,000,000				$ 501,000,000
Average price paid per share (in dollars per share)		$ 39.41	$ 36.48	$ 35.20	$ 36.18	$ 43.29	$ 39.05	$ 39.83				$ 39.57
Authorized repurchase amount														$ 250,000,000
Subsequent Event [Member] | Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased (in shares)	1,183,469										11,433,644		92,746,494
Repurchases of common stock	$ 49,000,000										$ 429,000,000		$ 2,645,000,000
Average price paid per share (in dollars per share)	$ 41.29										$ 37.51		$ 28.52
Remaining capacity of shares repurchase program	$ 169,000,000										$ 169,000,000		$ 169,000,000
Chief Executive Officer [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares authorized to be repurchased (in shares)															297,131
Deferred compensation equity, shares to be paid in the future (in shares)															297,131
Deferred equity compensation (in shares)															297,131
General Counsel [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares authorized to be repurchased (in shares)															23,062
Deferred compensation equity, shares to be paid in the future (in shares)															23,062
Deferred equity compensation (in shares)															23,062