Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Detail of regulatory liabilities
Regulatory liabilities, current	$ 99,899	$ 145,766
Regulatory liabilities, non-current	948,916	994,152
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	279,976	277,554
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	29,899	39,746
Regulatory liabilities, non-current	223,145	240,367
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	32,662	34,100
Regulatory liabilities, non-current	123,913	179,521
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	4,368	3,604
Regulatory liabilities, non-current	108,827	97,175
Income taxes - change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	1,771	1,113
Regulatory liabilities, non-current	70,898	72,454
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	0	3,051
Regulatory liabilities, non-current	71,726	67,437
Renewable energy standard (b)
Detail of regulatory liabilities
Regulatory liabilities, current	26,809	43,773
Regulatory liabilities, non-current	0	4,365
Demand side management (b)
Detail of regulatory liabilities
Regulatory liabilities, current	0	6,079
Regulatory liabilities, non-current	20,472	19,115
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	15,287	13,678
Deferred fuel and purchased power (b) (c)
Detail of regulatory liabilities
Regulatory liabilities, current	0	9,688
Regulatory liabilities, non-current	0	0
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	2,063	2,062
Regulatory liabilities, non-current	8,895	6,001
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	18,248	8,920
Other
Detail of regulatory liabilities
Regulatory liabilities, current	2,327	2,550
Regulatory liabilities, non-current	$ 7,529	$ 7,565