Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Detail of regulatory liabilities
Regulatory liabilities, current	$ 165,876	$ 100,086
Regulatory liabilities, non-current	2,325,976	2,452,536
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	278,585	332,171
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	39,866	18,238
Regulatory liabilities, non-current	177,533	209,191
Other post retirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	37,864	37,642
Regulatory liabilities, non-current	125,903	151,985
Income taxes - deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	2,164	2,164
Regulatory liabilities, non-current	51,120	52,497
Income taxes - change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	2,769	2,573
Regulatory liabilities, non-current	70,069	70,537
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	6,503	6,924
Regulatory liabilities, non-current	57,002	62,132
Renewable energy program
Detail of regulatory liabilities
Regulatory liabilities, current	44,966	23,155
Regulatory liabilities, non-current	20	0
Demand side management
Detail of regulatory liabilities
Regulatory liabilities, current	14,604	3,066
Regulatory liabilities, non-current	4,123	4,921
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	1,278	0
Regulatory liabilities, non-current	17,228	16,897
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	4,423	4,423
Regulatory liabilities, non-current	6,581	10,988
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	1,858	1,858
Regulatory liabilities, non-current	17,871	18,921
Tax expense adjustor mechanism
Detail of regulatory liabilities
Regulatory liabilities, current	3,237	0
Regulatory liabilities, non-current	0	0
Other
Detail of regulatory liabilities
Regulatory liabilities, current	42	43
Regulatory liabilities, non-current	3,541	2,022
ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	1,272,709	1,266,104
FERC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Regulatory liabilities, current	6,302	0
Regulatory liabilities, non-current	$ 243,691	$ 254,170