Summary of Significant Accounting Policies (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Basic earnings per share – continuing operations:
Income from continuing operations attributable to common shareholders	$ 419	$ 378	$ 966	$ 1,056
Participating securities dividends	0	0	0	(1)
Income from continuing operations available to common shareholders	$ 419	$ 378	$ 966	$ 1,055
Weighted average common shares outstanding (in shares)	326,000,000	326,000,000	326,000,000	326,000,000
Basic earnings per share - continuing operations (in dollars per share)	$ 1.29	$ 1.16	$ 2.96	$ 3.24
Diluted earnings per share – continuing operations:
Income from continuing operations available to common shareholders	$ 419	$ 378	$ 966	$ 1,055
Income impact of assumed conversions	0	0	0	1
Income from continuing operations available to common shareholders and assumed conversions	$ 419	$ 378	$ 966	$ 1,056
Weighted average common shares outstanding (in shares)	326,000,000	326,000,000	326,000,000	326,000,000
Incremental shares from assumed conversions (in shares)	3,000,000	2,000,000	3,000,000	3,000,000
Adjusted weighted average shares - diluted (in shares)	329,000,000	328,000,000	329,000,000	329,000,000
Diluted earnings per share - continuing operations (in dollars per share)	$ 1.27	$ 1.15	$ 2.94	$ 3.21
Stock option awards
Diluted earnings per share – continuing operations:
Stock-based compensation awards excluded from the computation of diluted earnings per share (in shares)	42,890	2,054,876	166,057	2,054,876