Employee Benefit Benefits and Stock-Based Compensation - Non-cash compensation expensed and capitalized (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock-based compensation expense
Stock- based compensation expense	$ 136	$ 102	$ 68
Capitalized Noncash Compensation	(1)	(1)	(1)
Non-cash compensation expense	135	101	67
Share-based Compensation Arrangement by Share-based Payment Award Accelerated Compensation Cost	9	12	8
Outperform Stock Options
Stock-based compensation expense
Stock- based compensation expense	14	10	10
Restricted Stock Units and Shares
Stock-based compensation expense
Stock- based compensation expense	40	22	19
401(k) Match Expense
Stock-based compensation expense
Stock- based compensation expense	23	13	11
Restricted Stock Unit Bonus Grant
Stock-based compensation expense
Stock- based compensation expense	46	57	28
Management Incentive and Retention Plan
Stock-based compensation expense
Stock- based compensation expense	$ 13	$ 0	$ 0