Fair Value Measurements (Details 2) (Level 3, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Level 3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 2	$ 2
Issuance of guarantees	1	0
Expiration of guarantees	(1)	0
Balance at end of period	$ 2	$ 2