Summary of Significant Accounting Policies (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic earnings per share – continuing operations:
Income from continuing operations attributable to common shareholders	$ 470	$ 421	$ 1,110	$ 983
Participating securities dividends	0	0	0	0
Income from continuing operations available to common shareholders	$ 470	$ 421	$ 1,110	$ 983
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)	$ 1.44	$ 1.29	$ 3.41	$ 3.01
Diluted earnings per share – continuing operations:
Income from continuing operations attributable to common shareholders	$ 470	$ 421	$ 1,110	$ 983
Participating securities dividends	0	0	0	0
Income from continuing operations available to common shareholders	470	421	1,110	983
Income impact of assumed conversions	0	0	0	0
Income from continuing operations available to common shareholders and assumed conversions	$ 470	$ 421	$ 1,110	$ 983
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)	2,000,000	4,000,000	3,000,000	4,000,000
Adjusted weighted average shares - diluted (in shares)	328,000,000	330,000,000	329,000,000	330,000,000
Diluted earnings per share - continuing operations (in dollars per share)	$ 1.43	$ 1.27	$ 3.38	$ 2.98
Stock option awards
Diluted earnings per share – continuing operations:
Stock-based compensation awards excluded from the computation of diluted earnings per share (in shares)	1,365,671	42,890	1,373,736	166,057