Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Regulatory Assets [Line Items]
Current regulatory assets	$ 1,300	$ 2,549
Noncurrent regulatory assets	58,141	60,298
Total regulatory assets	59,441	62,847
Regulatory Liabilities [Line Items]
Current regulatory liabilities	638	0
Noncurrent regulatory liabilities	19,252	17,395
Total regulatory liabilities	19,890	17,395
Postretirement benefits [Member]
Regulatory Liabilities [Line Items]
Noncurrent regulatory liabilities	19,047	17,395
Other [Member]
Regulatory Liabilities [Line Items]
Noncurrent regulatory liabilities	205	0
Environmental costs [Member]
Regulatory Assets [Line Items]
Current regulatory assets	1,300	1,300
Noncurrent regulatory assets	2,843	739
Fuel recovery [Member]
Regulatory Assets [Line Items]
Current regulatory assets	0	1,249
Grossed-up deferred taxes on equity funds used during construction [Member]
Regulatory Assets [Line Items]
Noncurrent regulatory assets	15,325	16,083
Levelized depreciation [Member]
Regulatory Assets [Line Items]
Noncurrent regulatory assets	33,261	33,979
Asset retirement obligations, net [Member]
Regulatory Assets [Line Items]
Noncurrent regulatory assets	$ 6,712	$ 9,497