Summary of Significant Accounting Policies (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Basic earnings per share – continuing operations:
Income from continuing operations attributable to common shareholders	$ 270	$ 299
Participating securities dividends	0	0
Income from continuing operations available to common shareholders	$ 270	$ 299
Weighted average common shares outstanding (in shares)	326,000,000	326,000,000
Basic earnings per share – continuing operations (in dollars per share)	$ 0.83	$ 0.92
Diluted earnings per share – continuing operations:
Income from continuing operations available to common shareholders	$ 270	$ 299
Income impact of assumed conversions	0	0
Income from continuing operations available to common shareholders and assumed conversions	$ 270	$ 299
Weighted average common shares outstanding (in shares)	326,000,000	326,000,000
Incremental shares from assumed conversions (in shares)	2,000,000	3,000,000
Adjusted weighted average shares - diluted (in shares)	328,000,000	329,000,000
Diluted earnings per share – continuing operations (in dollars per share)	$ 0.82	$ 0.91
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)	2,023,787	79,394