Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Detail of regulatory liabilities
Regulatory liabilities, current	$ 159,788	$ 100,086
Regulatory liabilities, non-current	2,410,597	2,452,536
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	344,402	332,171
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	30,871	18,238
Regulatory liabilities, non-current	196,065	209,191
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	37,842	37,642
Regulatory liabilities, non-current	123,683	151,985
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	2,137	2,164
Regulatory liabilities, non-current	50,555	52,497
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	2,799	2,573
Regulatory liabilities, non-current	71,137	70,537
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	7,769	6,924
Regulatory liabilities, non-current	57,400	62,132
Renewable energy standard
Detail of regulatory liabilities
Regulatory liabilities, current	41,912	23,155
Regulatory liabilities, non-current	92	0
Demand side management
Detail of regulatory liabilities
Regulatory liabilities, current	16,099	3,066
Regulatory liabilities, non-current	4,124	4,921
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	18,104	16,897
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	4,423	4,423
Regulatory liabilities, non-current	7,704	10,988
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	1,858	1,858
Regulatory liabilities, non-current	17,972	18,921
Tax expense adjustor mechanism
Detail of regulatory liabilities
Regulatory liabilities, current	7,433	0
Regulatory liabilities, non-current	0	0
Other
Detail of regulatory liabilities
Regulatory liabilities, current	361	43
Regulatory liabilities, non-current	2,837	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	1,273,153	1,266,104
United States Federal Energy Regulatory Commission | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Regulatory liabilities, current	6,284	0
Regulatory liabilities, non-current	$ 243,369	$ 254,170