Stock-Based Compensation (Details 2) (Long-term stock awards, USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Long-term stock awards
Unvested stock award shares
Balance at the beginning of the period (in shares)	6,000,000	8,000,000
Granted (in shares)	675,040	1,000,000
Vested (in shares)	2,000,000	2,000,000
Balance at the end of the period (in shares)	5,000,000	7,000,000
Weighted average grant date fair value
Balance at the beginning of the period (in dollars per share)	$ 18	$ 17
Granted (in dollars per share)	$ 26	$ 22
Vested (in dollars per share)	$ 17	$ 17
Forfeited (in dollars per share)	$ 19	$ 18
Balance at the end of the period (in dollars per share)	$ 19	$ 18
Additional disclosures
Total unrecognized compensation expense	$ 68	$ 94
Remaining weighted average vesting period	3 years	4 years
Total market value (at the vesting date) of stock award shares	$ 48	$ 45