Earnings Per Common Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator (basic and diluted):
Income from continuing operations	$ 111	$ 533	$ 281	$ 735
Less: Allocation to unvested restricted stock awards	2	11	4	15
Income from continuing operations attributable to common shareholders	109	522	277	720
Income (loss) from discontinued operations, net		10	(1)	21
Income (loss) from discontinued operations attributable to common shareholders		10	(1)	21
Net income available to common shareholders	$ 109	$ 532	$ 276	$ 741
Denominator:
Basic common shares (based upon weighted average) (in shares)	335	350	340	350
Add: Stock option dilution (in shares)	3	3	4	3
Diluted common shares (in shares)	338	353	344	353