Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Detail of regulatory liabilities
Regulatory liabilities, current	$ 116,172	$ 145,766
Regulatory liabilities, non-current	1,010,821	994,152
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	299,713	277,554
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	26,373	39,746
Regulatory liabilities, non-current	245,777	240,367
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	33,294	34,100
Regulatory liabilities, non-current	155,279	179,521
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	3,774	3,604
Regulatory liabilities, non-current	95,877	97,175
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	1,771	1,113
Regulatory liabilities, non-current	71,257	72,454
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	31	3,051
Regulatory liabilities, non-current	71,342	67,437
Renewable energy standard (b)
Detail of regulatory liabilities
Regulatory liabilities, current	35,882	43,773
Regulatory liabilities, non-current	2,182	4,365
Demand side management (b)
Detail of regulatory liabilities
Regulatory liabilities, current	4,957	6,079
Regulatory liabilities, non-current	21,864	19,115
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	14,483	13,678
Deferred fuel and purchased power (b) (c)
Detail of regulatory liabilities
Regulatory liabilities, current	2,439	9,688
Regulatory liabilities, non-current	0	0
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	2,062	2,062
Regulatory liabilities, non-current	9,535	6,001
Transmission cost adjustor (b)
Detail of regulatory liabilities
Regulatory liabilities, current	5,545	0
Regulatory liabilities, non-current	0	0
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	15,969	8,920
Other
Detail of regulatory liabilities
Regulatory liabilities, current	44	2,550
Regulatory liabilities, non-current	$ 7,543	$ 7,565