Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Detail of regulatory liabilities
Regulatory liabilities, current	$ 100,086	$ 99,899
Regulatory liabilities, non-current	2,452,536	948,916
Excess deferred income taxes - Tax Cuts and Jobs Act
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	1,520,274	0
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	332,171	279,976
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	18,238	29,899
Regulatory liabilities, non-current	209,191	223,145
Other post retirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	37,642	32,662
Regulatory liabilities, non-current	151,985	123,913
Income taxes - deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	2,164	4,368
Regulatory liabilities, non-current	52,497	108,827
Income taxes - change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	2,573	1,771
Regulatory liabilities, non-current	70,537	70,898
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	6,924	0
Regulatory liabilities, non-current	62,132	71,726
Renewable energy program
Detail of regulatory liabilities
Regulatory liabilities, current	23,155	26,809
Regulatory liabilities, non-current	0	0
Demand side management
Detail of regulatory liabilities
Regulatory liabilities, current	3,066	0
Regulatory liabilities, non-current	4,921	20,472
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	16,897	15,287
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	4,423	2,063
Regulatory liabilities, non-current	10,988	8,895
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	1,858	0
Regulatory liabilities, non-current	18,921	18,248
Other
Detail of regulatory liabilities
Regulatory liabilities, current	43	2,327
Regulatory liabilities, non-current	$ 2,022	$ 7,529