Retirement Plans and Other Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Retirement Plans and Other Benefits
Amount of pension and other postretirement benefit costs deferred		$ 5		$ 14
Amortization of regulatory assets	2		4
Pension Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost - benefits earned during the period	15	15	32	32
Interest cost on benefit obligation	28	30	56	60
Expected return on plan assets	(36)	(35)	(73)	(71)
Amortization of prior service cost			1	1
Amortization of net actuarial loss	10	12	20	22
Net periodic benefit cost	17	22	36	44
Portion of cost charged to expense	9	8	19	13
Contribution to pension plans			87
Pension Benefits | Expected contributions
Contributions
Expected contributions in 2013	140		140
Expected contributions in 2014	175		175
Expected contributions in 2015	175		175
Pension Benefits | Estimated contributions
Contributions
Expected contributions in 2013	0		0
Expected contributions in 2014	89		89
Expected contributions in 2015	112		112
Other Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost - benefits earned during the period	6	6	12	14
Interest cost on benefit obligation	10	11	20	23
Expected return on plan assets	(12)	(11)	(23)	(23)
Amortization of net actuarial loss	3	4	6	10
Net periodic benefit cost	7	10	15	24
Portion of cost charged to expense	4	3	9	6
Contributions
Expected contributions in 2013	20		20
Expected contributions in 2014	20		20
Expected contributions in 2015	$ 20		$ 20