Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Detail of regulatory liabilities
Regulatory liabilities, current	$ 120,671	$ 99,899
Regulatory liabilities, non-current	891,715	948,916
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	313,189	279,976
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	37,756	29,899
Regulatory liabilities, non-current	184,512	223,145
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	32,725	32,662
Regulatory liabilities, non-current	99,626	123,913
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	4,315	4,368
Regulatory liabilities, non-current	106,557	108,827
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	2,565	1,771
Regulatory liabilities, non-current	67,136	70,898
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	6,562	0
Regulatory liabilities, non-current	64,504	71,726
Renewable energy standard (c)
Detail of regulatory liabilities
Regulatory liabilities, current	17,915	26,809
Regulatory liabilities, non-current	0	0
Demand side management (c)
Detail of regulatory liabilities
Regulatory liabilities, current	12,175	0
Regulatory liabilities, non-current	4,921	20,472
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	16,494	15,287
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	4,525	2,063
Regulatory liabilities, non-current	11,875	8,895
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	1,857	0
Regulatory liabilities, non-current	19,494	18,248
Other
Detail of regulatory liabilities
Regulatory liabilities, current	276	2,327
Regulatory liabilities, non-current	$ 3,407	$ 7,529