Summary of Significant Accounting Policies (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 – continuing operations:
Income from continuing operations, net of tax									$ 985	$ 1,427	$ 879
Participating securities dividends									(1)	(1)	0
Income from continuing operations available to common shareholders									$ 984	$ 1,426	$ 879
Weighted average common shares outstanding (in shares)									326	326	326
Basic earnings (loss) per share – continuing operations (in dollars per share)	$ (0.22)	$ 1.16	$ 1.16	$ 0.92	$ 1.17	$ 1.52	$ 1.08	$ 0.61	$ 3.02	$ 4.38	$ 2.70
Diluted earnings per share – continuing operations:
Income from continuing operations available to common shareholders									$ 984	$ 1,426	$ 879
Income impact of assumed conversions									1	1	1
Income from continuing operations available to common shareholders and assumed conversions									$ 985	$ 1,427	$ 880
Weighted average common shares outstanding (in shares)									326	326	326
Incremental shares from assumed conversions (in shares)									3	3	3
Adjusted weighted average shares - diluted (in shares)									329	329	329
Diluted earnings (loss) per share – continuing operations (in dollars per share)	$ (0.22)	$ 1.15	$ 1.15	$ 0.91	$ 1.15	$ 1.51	$ 1.07	$ 0.61	$ 2.99	$ 4.33	$ 2.67