Variable Interest Entities (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Operating revenues	$ 335,875,000	$ 340,632,000	$ 1,170,829,000	$ 1,136,474,000
Lancaster Power Purchase Agreement [Member]
Evaluated power capacity, megawatts			270
Future contractual obligation	303,000,000		303,000,000
Palouse Wind Power Purchase Agreement [Member]
Evaluated power capacity, megawatts			105
Expected power production, in megawatts			40
Future contractual obligation	$ 562,000,000		$ 562,000,000
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