Stock-Based Compensation - Non-cash compensation expense and capitalized non-cash compensation (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
Non-cash compensation expense and capitalized non-cash compensation
Nonvested restricted stock and restricted stock units (RSUs)	4		4
OSOs outstanding	1		1
Stock-based compensation expense	$ 34	$ 22	$ 93	$ 48
Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount	0	0	(1)	0
Non-cash compensation expense and capitalized non-cash compensation	34	22	92	48
Outperform Stock Options
Non-cash compensation expense and capitalized non-cash compensation
Stock-based compensation expense	1	2	4	6
Restricted Stock Units and Shares
Non-cash compensation expense and capitalized non-cash compensation
Stock-based compensation expense	15	10	37	22
Performance Restricted Stock Units [Member]
Non-cash compensation expense and capitalized non-cash compensation
Stock-based compensation expense	10	4	23	7
401(k) Match Expense
Non-cash compensation expense and capitalized non-cash compensation
Stock-based compensation expense	8	6	29	18
Restricted Stock Unit Bonus Grant
Non-cash compensation expense and capitalized non-cash compensation
Stock-based compensation expense	$ 0	$ 0	$ 0	$ (5)