Summary of Significant Accounting Policies (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Basic earnings per share – continuing operations:
Income from continuing operations attributable to common shareholders	$ 278	$ 282	$ 640	$ 562
Participating securities dividends	0	0	0	0
Income from continuing operations available to common shareholders	$ 278	$ 282	$ 640	$ 562
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)	$ 0.85	$ 0.87	$ 1.96	$ 1.72
Diluted earnings per share – continuing operations:
Income from continuing operations attributable to common shareholders	$ 278	$ 282	$ 640	$ 562
Participating securities dividends	0	0	0	0
Income from continuing operations available to common shareholders	278	282	640	562
Income impact of assumed conversions	0	0	0	0
Income from continuing operations available to common shareholders and assumed conversions	$ 278	$ 282	$ 640	$ 562
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	4,000,000	3,000,000	4,000,000
Adjusted weighted average shares - diluted (in shares)	329,000,000	330,000,000	329,000,000	330,000,000
Diluted earnings per share - continuing operations (in dollars per share)	$ 0.85	$ 0.86	$ 1.95	$ 1.70
Stock option awards
Diluted earnings per share – continuing operations:
Stock-based compensation awards excluded from the computation of diluted earnings per share (in shares)	1,327,310	42,890	1,370,200	47,861