Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Current regulatory assets	$ 4,209	$ 3,299
Noncurrent regulatory assets	40,853	49,923
Total regulatory assets	45,062	53,222
Regulatory Liabilities [Line Items]
Noncurrent regulatory liabilities	26,802	23,375
Total regulatory liabilities	26,802	27,379
Fuel recovery [Member]
Regulatory Liabilities [Line Items]
Current regulatory liabilities	0	4,004
Postretirement benefits [Member]
Regulatory Liabilities [Line Items]
Noncurrent regulatory liabilities	26,802	23,375
Environmental costs [Member]
Regulatory Assets [Line Items]
Current regulatory assets	1,300	1,300
Noncurrent regulatory assets	(1,068)	2,109
Levelized depreciation [Member]
Regulatory Assets [Line Items]
Current regulatory assets	2,731	1,999
Noncurrent regulatory assets	26,493	29,775
Fuel Recovery [Member]
Regulatory Assets [Line Items]
Current regulatory assets	178	0
Grossed-up deferred taxes on equity funds used during construction [Member]
Regulatory Assets [Line Items]
Noncurrent regulatory assets	13,809	14,567
Asset retirement obligations, net [Member]
Regulatory Assets [Line Items]
Noncurrent regulatory assets	$ 1,619	$ 3,472