Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Fair Value Assets Measured on Recurring Basis Unobservable Input Reconciliation Asset
Level 3 measurement reconciliation, recurring basis, fair value assets beginning balance	$ 16	$ 23
Total unrealized gains included in other comprehensive income	2	3
Settlements, assets	(10)	(10)
Level 3 measurement reconciliation, recurring basis, fair value assets ending balance	8	16
Percentage of assets measured on recurring basis at fair value using significant unobservable inputs	1.00%
Auction rate securities
Fair Value Assets Measured on Recurring Basis Unobservable Input Reconciliation Asset
Level 3 measurement reconciliation, recurring basis, fair value assets beginning balance	16	23
Total unrealized gains included in other comprehensive income	2	3
Settlements, assets	(10)	(10)
Level 3 measurement reconciliation, recurring basis, fair value assets ending balance	$ 8	$ 16