Summary of Significant Accounting Policies (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic earnings per share – continuing operations:
Income from continuing operations attributable to common shareholders	$ 218	$ 362
Participating securities dividends	0	0
Income from continuing operations available to common shareholders	$ 218	$ 362
Weighted average common shares outstanding (in shares)	326,000,000	326,000,000
Basic earnings per share - continuing operations (in dollars per share)	$ 0.67	$ 1.11
Diluted earnings per share – continuing operations:
Income from continuing operations attributable to common shareholders	$ 218	$ 362
Participating securities dividends	0	0
Income from continuing operations available to common shareholders	218	362
Income impact of assumed conversions	0	0
Income from continuing operations available to common shareholders and assumed conversions	$ 218	$ 362
Weighted average common shares outstanding (in shares)	326,000,000	326,000,000
Incremental shares from assumed conversions (in shares)	1,000,000	3,000,000
Adjusted weighted average shares - diluted (in shares)	327,000,000	329,000,000
Diluted earnings per share - continuing operations (in dollars per share)	$ 0.67	$ 1.10
Stock option awards
Diluted earnings per share – continuing operations:
Stock-based compensation awards excluded from the computation of diluted earnings per share (in shares)	6,222,294	1,355,930