Common Stock - Summary of Stock Option Activity (Details) (Employee Stock Option [Member], USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
December 31, 2013	21,035,445
Options exercised	(291,805)
Options forfeited	(8,886)
Options expired	(1,903,787)
December 31, 2014	18,830,967
Weighted Average Exercise Price (in usd per share)	$ 46.85		$ 46.07
Options Reinstated, Weighted Average Exercise Price (in usd per share)	$ 25.27
Options Forfeited, Weighted Average Exercise Price (in usd per share)	$ 55.78
Options Expired, Weighted Average Exercise Price (in usd per share)	$ 41.47
Shares Outstanding, Weighted Average Remaining Contractual Life	4 years 1 month 10 days
Shares Outstanding, Aggregate Intrinsic Value	$ 29
Exercisable at December 31, 2014	18,398,932	[1]
Shares Exercisable, Weighted Average Exercise Price (in usd per share)	$ 47.01	[1]
Shares Exercisable, Weighted Average Remaining Contractual Life	4 years 15 days	[1]
Shares Exercisable, Aggregate Intrinsic Value	$ 29	[1]

[1] Includes stock options issued to retirement eligible employees.