Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Detail of regulatory liabilities
Regulatory liabilities, current	$ 260,404	$ 165,876
Regulatory liabilities, non-current	2,272,082	2,325,976
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	337,844	278,585
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	50,701	39,866
Regulatory liabilities, non-current	156,578	177,533
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	37,864	37,864
Regulatory liabilities, non-current	116,478	125,903
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	2,164	2,164
Regulatory liabilities, non-current	51,027	51,120
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	2,764	2,769
Regulatory liabilities, non-current	69,954	70,069
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	7,190	6,503
Regulatory liabilities, non-current	54,866	57,002
Renewable energy standard
Detail of regulatory liabilities
Regulatory liabilities, current	47,943	44,966
Regulatory liabilities, non-current	0	20
Demand side management
Detail of regulatory liabilities
Regulatory liabilities, current	1,581	14,604
Regulatory liabilities, non-current	24,146	4,123
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	6,657	1,278
Regulatory liabilities, non-current	11,637	17,228
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	3,923	4,423
Regulatory liabilities, non-current	5,975	6,581
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	1,858	1,858
Regulatory liabilities, non-current	17,690	17,871
Tax expense adjustor mechanism
Detail of regulatory liabilities
Regulatory liabilities, current	14	3,237
Regulatory liabilities, non-current	0	0
Other
Detail of regulatory liabilities
Regulatory liabilities, current	42	42
Regulatory liabilities, non-current	4,253	3,541
ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Regulatory liabilities, current	91,401	0
Regulatory liabilities, 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
Regulatory liabilities, current	6,302	6,302
Regulatory liabilities, non-current	$ 243,418	$ 243,691