Fair Value Measurements - Level 3 Rollforward Derivatives (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Net derivative beginning balance	$ (49,000)	$ (49,000)	$ (41,000)	$ (49,000)
Deferred as a regulatory asset or liability	6,000	3,000	(5,000)	6,000
Settlements	5,000	4,000	5,000	5,000
Transfers into Level 3 from Level 2	(4,000)	1,000	(4,000)	(2,000)
Transfers from Level 3 into Level 2	(1,000)	0	2,000	(1,000)
Net derivative ending balance	(43,000)	(41,000)	(43,000)	(41,000)
Net unrealized gains included in earnings related to instruments still held at end of period	$ 0	$ 0	$ 0	$ 0