Retirement Plans and Other Benefits - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2012	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Regulatory Assets
Regulatory asset amortization period	3 years
Amortization of regulatory asset		$ 2,000,000	$ 2,000,000	$ 4,000,000	$ 4,000,000
Pension Benefits
Contributions
Voluntary employer contributions to pension plan				80,000,000
Minimum employer contributions for the next three years				0
Maximum employer contributions for the next three years				300,000,000
Other Benefits
Other Postretirement Benefit Plan Remeasurement
Other postretirement plan benefit remeasurement, amount seeking approval to move to separate account to Pay Union employee medical costs				100,000,000
Contributions
2015				1,000,000
2016				1,000,000
2017				1,000,000
Maximum | Pension Benefits
Contributions
2015				100,000,000
2016				100,000,000
2017				$ 100,000,000