Retirement Plans and Other Benefits - Weighted-Average Assumptions for Pensions and Other Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Weighted-average assumptions used to determine benefit obligations
Rate of compensation increase (as a percent)	4.00%		4.00%	4.00%
Initial pre-65 healthcare cost trend rate (as a percent)			7.00%	7.50%
Initial post-65 healthcare cost trend rate (as a percent)			5.00%	7.50%
Ultimate health care cost trend rate (as a percent)			5.00%	5.00%
Number of years to ultimate trend rate (pre-65 participants)			4 years	4 years
Number of years to ultimate trend rate (post-65 participants)			0 years	4 years
Weighted-average assumptions used to determine net periodic benefit costs
Initial pre-65 health care cost trend rate (as a percent)	7.50%	7.50%		7.50%	7.50%
Initial post-65 health care cost trend rate (as a percent)	5.00%	7.50%		7.50%	7.50%
Ultimate healthcare cost trend rate (as a percent)	5.00%	5.00%		5.00%	5.00%
Number of years to ultimate trend rate (pre-65 participants)	4 years	4 years		4 years	4 years
Number of years to ultimate trend rate (post-65 participants)	0 years	4 years		4 years	4 years
Pension Benefits
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)	4.02%		4.02%	4.88%
Weighted-average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	4.88%	4.88%		4.01%	4.42%
Rate of compensation increase (as a percent)	4.00%	4.00%		4.00%	4.00%
Expected long-term return on plan assets (as a percent)	6.90%	6.90%		7.00%	7.75%
Other Benefits
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)	4.14%		4.14%	5.10%
Weighted-average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	4.41%	5.10%		4.20%	4.59%
Expected long-term return on plan assets (as a percent)	4.25%	6.80%		7.00%	7.75%
Effects of one percentage point change in the assumed initial and ultimate health care cost trend rates
Effect of 1% increase on other postretirement benefits expense, after consideration of amounts capitalized or billed to electric plant participants			10
Effect of 1% decrease on other postretirement benefits expense, after consideration of amounts capitalized or billed to electric plant participants			(4)
Effect of 1% increase on service and interest cost components of net periodic other postretirement benefit costs			12
Effect of 1% decrease on service and interest cost components of net periodic other postretirement benefit costs			(9)
Effect of 1% increase on the accumulated other postretirement benefit obligation			110
Effect of 1% decrease on the accumulated other postretirement benefit obligation			(88)