Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Detail of regulatory liabilities
Regulatory liabilities, current	$ 145,766	$ 130,549
Regulatory liabilities, non-current	994,152	1,051,196
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	277,554	295,546
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	39,746	31,033
Regulatory liabilities, non-current	240,367	272,825
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	34,100	32,317
Regulatory liabilities, non-current	179,521	198,599
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	3,604	3,505
Regulatory liabilities, non-current	97,175	92,727
Income taxes - change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	1,113	371
Regulatory liabilities, non-current	72,454	72,423
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	3,051	4,396
Regulatory liabilities, non-current	67,437	65,594
Renewable energy standard (b)
Detail of regulatory liabilities
Regulatory liabilities, current	43,773	24,596
Regulatory liabilities, non-current	4,365	22,677
Demand side management (b)
Detail of regulatory liabilities
Regulatory liabilities, current	6,079	31,335
Regulatory liabilities, non-current	19,115	0
Sundance maintenance
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	13,678	12,069
Deferred fuel and purchased power (b) (c)
Detail of regulatory liabilities
Regulatory liabilities, current	9,688	0
Regulatory liabilities, non-current	0	0
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	2,000	2,000
Regulatory liabilities, non-current	6,000	8,000
Four Corners coal reclamation
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	8,920	1,200
Other
Detail of regulatory liabilities
Regulatory liabilities, current	2,550	934
Regulatory liabilities, non-current	$ 7,565	$ 9,535