Fair Value Measurements - Level 3 Rollforward Derivatives (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Net derivative balance at beginning of period	$ (18,256)	$ (47,406)
Deferred as a regulatory asset or liability	(2,322)	(11,755)
Settlements	782	1,423
Transfers into Level 3 from Level 2	(2,445)	(38)
Transfers from Level 3 into Level 2	2,487	16,091
Net derivative balance at end of period	(19,754)	(41,685)
Net unrealized gains included in earnings related to instruments still held at end of period	$ 0	$ 0