Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Detail of regulatory liabilities
Current	$ 208,022	$ 165,876
Non-Current	2,310,131	2,325,976
Asset retirement obligations
Detail of regulatory liabilities
Current	0	0
Non-Current	367,930	278,585
Removal costs
Detail of regulatory liabilities
Current	47,459	39,866
Non-Current	151,535	177,533
Other postretirement benefits
Detail of regulatory liabilities
Current	37,821	37,864
Non-Current	95,789	125,903
Income taxes — change in rates
Detail of regulatory liabilities
Current	2,764	2,769
Non-Current	67,605	70,069
Spent nuclear fuel
Detail of regulatory liabilities
Current	5,746	6,503
Non-Current	53,229	57,002
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Current	2,164	2,164
Non-Current	49,182	51,120
Four Corners coal reclamation
Detail of regulatory liabilities
Current	1,858	1,858
Non-Current	49,194	17,871
Renewable energy standard
Detail of regulatory liabilities
Current	42,146	44,966
Non-Current	5,675	20
Demand side management
Detail of regulatory liabilities
Current	14,300	14,604
Non-Current	24,146	4,123
Sundance maintenance
Detail of regulatory liabilities
Current	4,640	1,278
Non-Current	13,393	17,228
Deferred gains on utility property
Detail of regulatory liabilities
Current	2,923	4,423
Non-Current	4,766	6,581
Deferred property taxes
Detail of regulatory liabilities
Current	0	0
Non-Current	6,288	2,611
FERC transmission true up
Detail of regulatory liabilities
Current	0	0
Non-Current	2,586	0
Other
Detail of regulatory liabilities
Current	1,370	3,279
Non-Current	2,533	930
ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Current	38,529	0
Non-Current	1,178,216	1,272,709
United States Federal Energy Regulatory Commission | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Current	6,302	6,302
Non-Current	$ 238,064	$ 243,691