Retirement Plans and Other Benefits (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		1 Months Ended	3 Months Ended
	Jul. 31, 2012	Mar. 31, 2014	Mar. 31, 2013	Jan. 31, 2014 Pension Benefits	Mar. 31, 2014 Pension Benefits	Mar. 31, 2013 Pension Benefits	Mar. 31, 2014 Pension Benefits Expected contributions	Mar. 31, 2014 Pension Benefits Expected contributions Maximum	Mar. 31, 2014 Other Benefits	Mar. 31, 2013 Other Benefits	Mar. 31, 2014 Other Benefits Expected contributions
Retirement Plans and Other Benefits
Regulatory asset amortization period	3 years
Amortization of regulatory assets		$ 2	$ 2
Net periodic benefit costs and the portion of these costs charged to expense
Service cost - benefits earned during the period					15	17			5	6
Interest cost on benefit obligation					32	29			11	10
Expected return on plan assets					(40)	(37)			(12)	(11)
Amortization of net actuarial loss					2	9				3
Net periodic benefit cost					9	18			4	8
Portion of cost charged to expense					5	10			3	5
Contributions
Employer's contributions under the plan				105
Minimum Contributions under MAP-21
Minimum contributions under MAP-21					141
2014					0
2015					19
2016					122
Expected employer's contributions
Expected employer's contributions							300
2014							175				10
2015								100			10
2016								$ 25			$ 10