Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Detail of regulatory liabilities
Regulatory liabilities, current	$ 136,535	$ 100,086
Regulatory liabilities, non-current	2,415,417	2,452,536
Excess deferred income taxes - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	1,519,224	1,520,274
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	315,922	332,171
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	26,949	18,238
Regulatory liabilities, non-current	197,274	209,191
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	37,642	37,642
Regulatory liabilities, non-current	142,560	151,985
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	2,144	2,164
Regulatory liabilities, non-current	52,478	52,497
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	2,799	2,573
Regulatory liabilities, non-current	73,703	70,537
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	6,609	6,924
Regulatory liabilities, non-current	61,736	62,132
Renewable energy standard (c)
Detail of regulatory liabilities
Regulatory liabilities, current	32,694	23,155
Regulatory liabilities, non-current	0	0
Demand side management (c)
Detail of regulatory liabilities
Regulatory liabilities, current	4,049	3,066
Regulatory liabilities, non-current	4,123	4,921
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	17,299	16,897
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	4,423	4,423
Regulatory liabilities, non-current	9,873	10,988
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	1,858	1,858
Regulatory liabilities, non-current	18,525	18,921
Tax expense adjustor mechanism
Detail of regulatory liabilities
Regulatory liabilities, current	15,676	0
Regulatory liabilities, non-current	0	0
Other
Detail of regulatory liabilities
Regulatory liabilities, current	1,692	43
Regulatory liabilities, non-current	$ 2,700	$ 2,022