Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Detail of regulatory liabilities
Current	$ 279,105	$ 234,912
Non-Current	2,067,801	2,267,835
Asset retirement obligations
Detail of regulatory liabilities
Current	0	0
Non-Current	311,517	418,423
Removal costs
Detail of regulatory liabilities
Current	44,586	47,356
Non-Current	135,450	136,072
Other postretirement benefits
Detail of regulatory liabilities
Current	37,575	37,575
Non-Current	130,270	139,634
Spent nuclear fuel
Detail of regulatory liabilities
Current	6,638	6,676
Non-Current	49,234	51,019
Four Corners coal reclamation
Detail of regulatory liabilities
Current	5,461	1,059
Non-Current	48,405	51,704
Income taxes — change in rates
Detail of regulatory liabilities
Current	2,802	2,797
Non-Current	51,152	68,265
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Current	2,202	2,202
Non-Current	49,910	50,034
Renewable energy standard
Detail of regulatory liabilities
Current	45,872	39,287
Non-Current	115	10,300
Demand side management
Detail of regulatory liabilities
Current	1,702	15,024
Non-Current	43,423	24,146
Sundance maintenance
Detail of regulatory liabilities
Current	184	5,698
Non-Current	13,515	11,319
Active union medical trust
Detail of regulatory liabilities
Current	0	0
Non-Current	7,986	2,041
Deferred property taxes
Detail of regulatory liabilities
Current	0	0
Non-Current	7,968	7,046
Tax expense adjustor mechanism
Detail of regulatory liabilities
Current	6,615	7,018
Non-Current	0	0
Deferred gains on utility property
Detail of regulatory liabilities
Current	2,423	2,423
Non-Current	3,577	4,163
FERC transmission true up
Detail of regulatory liabilities
Current	3,304	1,045
Non-Current	1,621	2,004
Other
Detail of regulatory liabilities
Current	284	532
Non-Current	132	255
ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Current	113,142	59,918
Non-Current	976,018	1,054,053
United States Federal Energy Regulatory Commission | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Current	6,315	6,302
Non-Current	$ 237,508	$ 237,357