Summary of Significant Accounting Policies (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic earnings per share – continuing operations:
Income from continuing operations attributable to common shareholders	$ 276	$ 278	$ 494	$ 640
Participating securities dividends	0	0	0	0
Income from continuing operations available to common shareholders	$ 276	$ 278	$ 494	$ 640
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	$ 0.85	$ 1.52	$ 1.96
Diluted earnings per share – continuing operations:
Income from continuing operations attributable to common shareholders	$ 276	$ 278	$ 494	$ 640
Participating securities dividends	0	0	0	0
Income from continuing operations available to common shareholders	276	278	494	640
Income impact of assumed conversions	0	0	0	0
Income from continuing operations available to common shareholders and assumed conversions	$ 276	$ 278	$ 494	$ 640
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)	1,000,000	3,000,000	1,000,000	3,000,000
Adjusted weighted average shares - diluted (in shares)	327,000,000	329,000,000	327,000,000	329,000,000
Diluted earnings per share - continuing operations (in dollars per share)	$ 0.84	$ 0.85	$ 1.51	$ 1.95
Stock option awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock-based compensation awards excluded from the computation of diluted earnings per share (in shares)	6,223,964	1,327,310	6,223,964	1,370,200