Variable Interest Entities (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating revenues	$ 447,676,000	$ 335,875,000	$ 352,048,000	$ 482,906,000	$ 410,528,000	$ 340,632,000	$ 343,585,000	$ 452,257,000	$ 1,618,505,000	$ 1,547,002,000	$ 1,619,780,000
Noncontrolling interest	(134,000)	518,000	73,000	760,000	235,000	176,000	354,000	(175,000)
Lancaster Power Purchase Agreement [Member]
Evaluated power capacity, megawatts									270
Future contractual obligation	298,000,000								298,000,000
Palouse Wind Power Purchase Agreement [Member]
Evaluated power capacity, megawatts									105
Expected power production, in megawatts									40
Future contractual obligation	$ 604,000,000								$ 604,000,000
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