Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Detail of regulatory liabilities
Regulatory liabilities, current	$ 234,912	$ 165,876
Regulatory liabilities, non-current	2,267,835	2,325,976
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	418,423	278,585
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	47,356	39,866
Regulatory liabilities, non-current	136,072	177,533
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	37,575	37,864
Regulatory liabilities, non-current	139,634	125,903
Income taxes - change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	2,797	2,769
Regulatory liabilities, non-current	68,265	70,069
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	6,676	6,503
Regulatory liabilities, non-current	51,019	57,002
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	1,059	1,858
Regulatory liabilities, non-current	51,704	17,871
Income taxes - deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	2,202	2,164
Regulatory liabilities, non-current	50,034	51,120
Renewable energy program
Detail of regulatory liabilities
Regulatory liabilities, current	39,287	44,966
Regulatory liabilities, non-current	10,300	20
Demand side management
Detail of regulatory liabilities
Regulatory liabilities, current	15,024	14,604
Regulatory liabilities, non-current	24,146	4,123
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	5,698	1,278
Regulatory liabilities, non-current	11,319	17,228
Property tax deferral
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	7,046	2,611
Tax expense adjustor mechanism
Detail of regulatory liabilities
Regulatory liabilities, current	7,018	3,237
Regulatory liabilities, non-current	0	0
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	2,423	4,423
Regulatory liabilities, non-current	4,163	6,581
FERC transmission true up
Detail of regulatory liabilities
Regulatory liabilities, current	1,045	0
Regulatory liabilities, non-current	2,004	0
Other
Detail of regulatory liabilities
Regulatory liabilities, current	532	42
Regulatory liabilities, non-current	2,296	930
ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Regulatory liabilities, current	59,918	0
Regulatory liabilities, non-current	1,054,053	1,272,709
FERC | 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	$ 237,357	$ 243,691