Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic Earnings per Share
Net Income	$ 80	$ 265	$ 109	$ 273	$ 299	$ 156	$ 124	$ 326	$ 727	$ 905	$ 661
Average number of common shares outstanding									179	177	175
Weighted average net restricted shares outstanding									0	0	1
Dividends declared — common shares									$ 508	$ 475	$ 453
Dividends declared — net restricted shares									2	1	1
Total distributed earnings									510	476	454
Net Income less distributed earnings									$ 217	$ 429	$ 207
Distributed (dividends per common share) (in dollars per share)									$ 2.84	$ 2.69	$ 2.59
Undistributed (in dollars per share)									1.21	2.42	1.17
Total Basic Earnings per Common Share (in dollars per share)	$ 0.45	$ 1.47	$ 0.61	$ 1.53	$ 1.68	$ 0.88	$ 0.70	$ 1.84	$ 4.05	$ 5.11	$ 3.76
Diluted Earnings per Share
Net Income	$ 80	$ 265	$ 109	$ 273	$ 299	$ 156	$ 124	$ 326	$ 727	$ 905	$ 661
Average number of common shares outstanding									179	177	175
Weighted average net restricted shares outstanding									0	0	1
Dividends declared — common shares									$ 508	$ 475	$ 453
Dividends declared — net restricted shares									2	1	1
Total distributed earnings									510	476	454
Net income less distributed earnings									$ 217	$ 429	$ 207
Distributed (dividends per common share) (in dollars per share)									$ 2.84	$ 2.69	$ 2.59
Undistributed (in dollars per share)									1.21	2.41	1.17
Total Diluted Earnings per Common Share (in dollars per share)	$ 0.45	$ 1.47	$ 0.61	$ 1.53	$ 1.68	$ 0.88	$ 0.70	$ 1.84	$ 4.05	$ 5.10	$ 3.76