Summary of Significant Accounting Policies (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Basic earnings per share – continuing operations:
Income from continuing operations attributable to common shareholders	$ 278	$ 379	$ 547	$ 678
Participating securities dividends	0	0	0	0
Income from continuing operations available to common shareholders	$ 278	$ 379	$ 547	$ 678
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)	$ 0.86	$ 1.16	$ 1.68	$ 2.08
Diluted earnings per share – continuing operations:
Income from continuing operations available to common shareholders	$ 278	$ 379	$ 547	$ 678
Income impact of assumed conversions	0	0	0	0
Income from continuing operations available to common shareholders and assumed conversions	$ 278	$ 379	$ 547	$ 678
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)	$ 0.85	$ 1.15	$ 1.66	$ 2.06
Stock-Based Compensation Awards
Diluted earnings per share – continuing operations:
Stock-based compensation awards excluded from the computation of diluted earnings per share (in shares)	42,890	1,981,596	47,861	1,981,596