Variable Interest Entities (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Operating revenues	$ 490,962,000	$ 482,906,000
Lancaster Power Purchase Agreement [Member]
Evaluated Power Capacity	270
Future contractual obligation	292,000,000
Palouse Wind Power Purchase Agreement [Member]
Evaluated Power Capacity	105
Expected power production, in megawatts	40
Future contractual obligation	$ 600,000,000
Minimum [Member] | Lancaster Power Purchase Agreement [Member]
Minimum estimated life of plant, in years	15 years
Maximum [Member] | Lancaster Power Purchase Agreement [Member]
Minimum estimated life of plant, in years	25 years