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, 2016	Dec. 31, 2015
Weighted-average assumptions used to determine benefit obligations
Rate of compensation increase (as a percent)			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%	7.00%
Initial post-65 health care cost trend rate (as a percent)	5.00%	7.50%	5.00%	5.00%
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	0 years
Pension Benefits
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)			4.08%	4.37%
Weighted-average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	4.88%	4.88%	4.37%	4.02%
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%	6.90%
Other Benefits
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)			4.17%	4.52%
Weighted-average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	4.41%	5.10%	4.52%	4.14%
Expected long-term return on plan assets (as a percent)	4.25%	6.80%	4.45%	4.45%
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			$ 8,430
Effect of 1% decrease on other postretirement benefits expense, after consideration of amounts capitalized or billed to electric plant participants			(5,455)
Effect of 1% increase on service and interest cost components of net periodic other postretirement benefit costs			8,440
Effect of 1% decrease on service and interest cost components of net periodic other postretirement benefit costs			(6,527)
Effect of 1% increase on the accumulated other postretirement benefit obligation			108,046
Effect of 1% decrease on the accumulated other postretirement benefit obligation			$ (86,651)