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Fair Value Measurements - Level 3 Rollforward Derivatives (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]        
Net derivative balance at beginning of period $ (36,245) $ (32,380) $ (47,406) $ (32,979)
Included in OCI (4) (10) (10) 94
Deferred as a regulatory asset or liability (3,769) (13,499) (11,272) (21,103)
Settlements 1,733 5,424 4,855 11,691
Transfers into Level 3 from Level 2 (5,952) 1,343 (10,340) 1,725
Transfers from Level 3 into Level 2 5,632 (420) 25,568 1,030
Net derivative balance at end of period (38,605) (39,542) (38,605) (39,542)
Net unrealized gains included in earnings related to instruments still held at end of period $ 0 $ 0 $ 0 $ 0