Employee Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Pension Plans [Member]
Components of net periodic benefit cost
Service cost	$ 32	$ 30
Interest cost	42	42
Expected return on plan assets	(48)	(52)
Prior service (credit) cost and transition (asset) obligation amortization	(1)	(2)
Actuarial (gain) loss amortization	40	31
Net periodic benefit cost	65	49
Postretirement Welfare Plan [Member]
Components of net periodic benefit cost
Service cost	1	1
Interest cost	2	2
Expected return on plan assets		(1)
Prior service (credit) cost and transition (asset) obligation amortization
Actuarial (gain) loss amortization	(2)	(1)
Net periodic benefit cost	$ 1	$ 1