Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Detail of regulatory liabilities
Regulatory liabilities, current	$ 152,000	$ 131,000
Regulatory liabilities, non-current	1,016,991	1,051,196
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	44,000	31,000
Regulatory liabilities, non-current	245,000	273,000
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	272,000	296,000
Renewable energy standard
Detail of regulatory liabilities
Regulatory liabilities, current	29,000	25,000
Regulatory liabilities, non-current	20,000	23,000
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	1,000	0
Regulatory liabilities, non-current	71,000	72,000
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	3,000	5,000
Regulatory liabilities, non-current	68,000	66,000
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	2,000	2,000
Regulatory liabilities, non-current	7,000	8,000
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	3,000	4,000
Regulatory liabilities, non-current	92,000	93,000
Deferred fuel and purchased power
Detail of regulatory liabilities
Regulatory liabilities, current	16,000	0
Regulatory liabilities, non-current	0	0
Demand side management
Detail of regulatory liabilities
Regulatory liabilities, current	8,000	31,000
Regulatory liabilities, non-current	27,000	0
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	33,000	32,000
Regulatory liabilities, non-current	189,000	199,000
Other
Detail of regulatory liabilities
Regulatory liabilities, current	13,000	1,000
Regulatory liabilities, non-current	$ 26,000	$ 21,000