Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Detail of regulatory liabilities
Regulatory liabilities, current	$ 104,313	$ 145,766
Regulatory liabilities, non-current	998,349	994,152
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	305,862	277,554
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	27,626	39,746
Regulatory liabilities, non-current	237,885	240,367
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	33,294	34,100
Regulatory liabilities, non-current	146,988	179,521
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	3,774	3,604
Regulatory liabilities, non-current	93,578	97,175
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	1,771	1,113
Regulatory liabilities, non-current	70,233	72,454
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	0	3,051
Regulatory liabilities, non-current	71,884	67,437
Renewable energy standard (b)
Detail of regulatory liabilities
Regulatory liabilities, current	28,921	43,773
Regulatory liabilities, non-current	1,091	4,365
Demand side management (b)
Detail of regulatory liabilities
Regulatory liabilities, current	4,261	6,079
Regulatory liabilities, non-current	21,863	19,115
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	14,885	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	9,335	6,001
Transmission cost adjustor (b)
Detail of regulatory liabilities
Regulatory liabilities, current	2,077	0
Regulatory liabilities, non-current	0	0
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	17,213	8,920
Other
Detail of regulatory liabilities
Regulatory liabilities, current	526	2,550
Regulatory liabilities, non-current	$ 7,532	$ 7,565