Fair value measurements (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Fair Value Assets Measured on Recurring Basis Unobservable Input Reconciliation Asset and Liability
Level 3 measurement reconciliation, recurring basis, fair value assets beginning balance	$ 16	$ 23
Total unrealized gains included in other comprehensive income	1	2
Level 3 measurement reconciliation, recurring basis, fair value assets ending balance	17	25
Percentage of assets measured on recurring basis at fair value using significant unobservable inputs	1.00%
ARS
Fair Value Assets Measured on Recurring Basis Unobservable Input Reconciliation Asset and Liability
Level 3 measurement reconciliation, recurring basis, fair value assets beginning balance	16	23
Total unrealized gains included in other comprehensive income	1	2
Level 3 measurement reconciliation, recurring basis, fair value assets ending balance	$ 17	$ 25