Commitments and Contingencies, Variable Interest Entities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) MW	Dec. 31, 2015 USD ($) MW
Purchased Power Agreements [Abstract]
VIE Purchase Power Agreement Expiration Date	2041
Independent Power Producing Entities
Purchased Power Agreements [Abstract]
Generating capacity (in MW) | MW	3,537	3,698
Harrington Station Power Plant [Member]
Fuel Contracts [Abstract]
Coal Supply Agreement Expiration Date	2017
Tolk Station Power Plant [Member]
Fuel Contracts [Abstract]
Coal Supply Agreement Expiration Date	2017
Low-Income Housing Limited Partnerships
Amount Reflected in Consolidated Balance Sheets [Abstract]
Current assets	$ 7,102	$ 6,274
Property, plant and equipment, net	49,638	51,480
Other noncurrent assets	918	977
Total assets	57,658	58,731
Current liabilities	7,769	7,540
Mortgages and other long-term debt payable	30,343	30,665
Other noncurrent liabilities	658	644
Total liabilities	$ 38,770	$ 38,849