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Fair Value Measurements - Level 3 Rollforward Derivatives (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Net derivative balance at beginning of period $ (47,406) $ (32,979)
Included in OCI 0 0
Deferred as a regulatory asset or liability (11,755) (9,103)
Settlements 1,423 1,765
Transfers into Level 3 from Level 2 (38) 262
Transfers from Level 3 into Level 2 16,091 548
Net derivative balance at end of period (41,685) (39,507)
Net unrealized gains included in earnings related to instruments still held at end of period $ 0 $ 0