Retirement Plans and Other Benefits - Weighted-Average Assumptions for Pensions and Other Benefits (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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%	7.00%
Initial post-65 healthcare cost trend rate (as a percent)			5.00%	5.00%
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	0 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%	7.00%		7.50%
Initial post-65 health care cost trend rate (as a percent)	5.00%	7.50%	5.00%		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	0 years		4 years
Pension Benefits
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)	4.02%		4.37%	4.02%
Weighted-average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	4.88%	4.88%	4.02%		4.01%
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%	6.90%		7.00%
Other Benefits
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)	4.14%		4.52%	4.14%
Weighted-average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	4.41%	5.10%	4.14%		4.20%
Expected long-term return on plan assets (as a percent)	4.25%	6.80%	4.45%		7.00%
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			$ 8,834
Effect of 1% decrease on other postretirement benefits expense, after consideration of amounts capitalized or billed to electric plant participants			(5,890)
Effect of 1% increase on service and interest cost components of net periodic other postretirement benefit costs			9,069
Effect of 1% decrease on service and interest cost components of net periodic other postretirement benefit costs			(6,949)
Effect of 1% increase on the accumulated other postretirement benefit obligation			100,322
Effect of 1% decrease on the accumulated other postretirement benefit obligation			$ (80,332)