Retirement Plans and Other Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Retirement Plans and Other Benefits
Amount of pension and other postretirement benefit costs deferred					$ 14
Amortization of regulatory assets	2	2	6	2
Pension Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost - benefits earned during the period	16	16	48	48
Interest cost on benefit obligation	28	30	84	90
Expected return on plan assets	(36)	(35)	(110)	(106)
Amortization of prior service cost			1	1
Amortization of net actuarial loss	10	11	30	33
Net periodic benefit cost	18	22	53	66
Portion of cost charged to expense	10	12	29	25
Contributions
Employer's contributions under the plan			141
Minimum Contributions under MAP-21
2013			0
2014			89
2015			112
Pension Benefits | Maximum
Expected employer's contributions
2014			175
2015			175
Other Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost - benefits earned during the period	6	7	18	20
Interest cost on benefit obligation	10	12	31	35
Expected return on plan assets	(11)	(12)	(34)	(34)
Amortization of net actuarial loss	3	5	8	15
Net periodic benefit cost	8	12	23	36
Portion of cost charged to expense	5	7	14	13
Contributions
Employer's contributions under the plan			11
Expected employer's contributions
2013			14
2014			20
2015			$ 20