Postretirement Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Components of net periodic benefit cost:
Service cost	$ 5	$ 4	$ 16	$ 13
Interest cost	6	6	18	17
Expected return on plan assets	(6)	(7)	(20)	(20)
Amortization of prior service cost	1	1	1	1
Amortization of actuarial loss	2	1	8	4
Net periodic benefit cost	$ 8	$ 5	$ 23	$ 15