Summary of Significant Accounting Policies (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic earnings per share – continuing operations:
Income from continuing operations, net of tax									$ 565	$ 1,299	$ 985
Participating securities dividends									0	0	(1)
Income from continuing operations available to common shareholders									$ 565	$ 1,299	$ 984
Weighted average common shares outstanding (in shares)									326,000,000	326,000,000	326,000,000
Basic earnings per share – continuing operations (in dollars per share)	$ (1.67)	$ 1.44	$ 0.85	$ 1.11	$ 0.97	$ 1.29	$ 0.87	$ 0.86	$ 1.73	$ 3.99	$ 3.02
Diluted earnings per share – continuing operations:
Income from continuing operations, net of tax									$ 565	$ 1,299	$ 985
Participating securities dividends									0	0	(1)
Income from continuing operations available to common shareholders									565	1,299	984
Income impact of assumed conversions									0	1	1
Income from continuing operations available to common shareholders and assumed conversions									$ 565	$ 1,300	$ 985
Weighted average common shares outstanding (in shares)									326,000,000	326,000,000	326,000,000
Incremental shares from assumed conversions (in shares)									2,000,000	4,000,000	3,000,000
Adjusted weighted average shares - diluted (in shares)									328,000,000	330,000,000	329,000,000
Diluted earnings per share – continuing operations (in dollars per share)	$ (1.66)	$ 1.43	$ 0.85	$ 1.10	$ 0.96	$ 1.27	$ 0.86	$ 0.85	$ 1.72	$ 3.94	$ 2.99
Stock Compensation Plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive stock option awards excluded from earnings per share, number of shares									1,334,451	167,795	2,046,045