Shareholders' Equity - Shares Repurchased (Details) - USD ($)	1 Months Ended	3 Months Ended							9 Months Ended		10 Months Ended	12 Months Ended	58 Months Ended
	Nov. 02, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Nov. 02, 2017	Dec. 31, 2016	Nov. 02, 2017	Nov. 01, 2017	Jan. 06, 2017
Equity, Class of Treasury Stock [Line Items]
Repurchases of common stock									$ 431,000,000	$ 190,000,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased (in shares)		1,847,901	3,456,711	5,430,041	3,300,617	2,050,229	2,331,474	3,038,928				10,721,248
Repurchases of common stock		$ 80,000,000	$ 135,000,000	$ 216,000,000	$ 116,000,000	$ 55,000,000	$ 60,000,000	$ 75,000,000				$ 306,000,000
Treasury Stock Acquired, Average Cost Per Share		$ 43.29	$ 39.05	$ 39.83	$ 35.09	$ 26.83	$ 25.73	$ 24.69				$ 28.53
Subsequent Event [Member] | Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased (in shares)	533,618										11,268,271		79,911,478
Repurchases of common stock	$ 20,000,000										$ 451,000,000		$ 2,166,000,000
Treasury Stock Acquired, Average Cost Per Share	$ 37.48										$ 40.05		$ 27.11
Remaining capacity of shares repurchase program	$ 398,000,000										$ 398,000,000		$ 398,000,000
Authorized repurchase amount														$ 300,000,000
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