Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Detail of regulatory liabilities
Current	$ 231,857	$ 165,876
Non-Current	2,287,892	2,325,976
Asset retirement obligations
Detail of regulatory liabilities
Current	0	0
Non-Current	359,912	278,585
Removal costs
Detail of regulatory liabilities
Current	50,701	39,866
Non-Current	153,847	177,533
Other postretirement benefits
Detail of regulatory liabilities
Current	37,821	37,864
Non-Current	105,214	125,903
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Current	2,164	2,164
Non-Current	50,414	51,120
Income taxes — change in rates
Detail of regulatory liabilities
Current	2,764	2,769
Non-Current	69,171	70,069
Spent nuclear fuel
Detail of regulatory liabilities
Current	6,578	6,503
Non-Current	53,769	57,002
Renewable energy standard
Detail of regulatory liabilities
Current	40,639	44,966
Non-Current	4,950	20
Demand side management
Detail of regulatory liabilities
Current	7,191	14,604
Non-Current	24,146	4,123
Sundance maintenance
Detail of regulatory liabilities
Current	1,641	1,278
Non-Current	15,495	17,228
Deferred gains on utility property
Detail of regulatory liabilities
Current	3,423	4,423
Non-Current	5,372	6,581
Four Corners coal reclamation
Detail of regulatory liabilities
Current	1,858	1,858
Non-Current	17,540	17,871
Tax expense adjustor mechanism
Detail of regulatory liabilities
Current	1,663	3,237
Non-Current	0	0
Other
Detail of regulatory liabilities
Current	461	42
Non-Current	8,213	3,541
ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act (a)
Detail of regulatory liabilities
Current	68,651	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 (a)
Detail of regulatory liabilities
Current	6,302	6,302
Non-Current	$ 241,633	$ 243,691