Variable Interest Entities (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Operating revenues	$ 352,048,000	$ 343,585,000	$ 834,954,000	$ 795,842,000
Lancaster Power Purchase Agreement [Member]
Evaluated power capacity, megawatts			270
Future contractual obligation	309,000,000		309,000,000
Palouse Wind Power Purchase Agreement [Member]
Evaluated power capacity, megawatts			105
Expected power production, in megawatts			40
Future contractual obligation	$ 567,000,000		$ 567,000,000
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