Variable Interest Entities (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) MW	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Operating revenues from continuing operations	$ 387,305	$ 313,649	$ 337,332	$ 446,490	$ 411,846	$ 301,558	$ 312,580	$ 446,578	$ 1,484,776	$ 1,472,562	$ 1,441,744
Noncontrolling interest	17	$ 32	$ 28	$ 13	$ 23	$ 20	$ (289)	$ 482
Lancaster Power Purchase Agreement [Member]
Evaluated Power Capacity | MW									270
Future contractual obligation	$ 296,500								$ 296,500
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