Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Detail of regulatory liabilities
Regulatory liabilities, current	$ 101,208	$ 99,899
Regulatory liabilities, non-current	948,293	948,916
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	298,796	279,976
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	37,194	29,899
Regulatory liabilities, non-current	211,348	223,145
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	32,662	32,662
Regulatory liabilities, non-current	115,950	123,913
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	4,315	4,368
Regulatory liabilities, non-current	108,691	108,827
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	2,565	1,771
Regulatory liabilities, non-current	69,497	70,898
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	72,755	71,726
Renewable energy standard (b)
Detail of regulatory liabilities
Regulatory liabilities, current	22,367	26,809
Regulatory liabilities, non-current	0	0
Demand side management (b)
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	19,921	20,472
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	15,690	15,287
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	2,062	2,063
Regulatory liabilities, non-current	8,439	8,895
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	19,684	18,248
Other
Detail of regulatory liabilities
Regulatory liabilities, current	43	2,327
Regulatory liabilities, non-current	$ 7,522	$ 7,529