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Fair Value Measurements - Level 3 Rollforward Derivatives (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]        
Net derivative balance at beginning of period $ (9,358) $ (36,245) $ (18,256) $ (47,406)
Included in OCI 0 (4) 0 (10)
Deferred as a regulatory asset or liability 1,244 (3,769) (2,067) (11,272)
Settlements (2,332) 1,733 (1,056) 4,855
Transfers into Level 3 from Level 2 (2,246) (5,952) (7,225) (10,340)
Transfers from Level 3 into Level 2 2,829 5,632 18,741 25,568
Net derivative balance at end of period (9,863) (38,605) (9,863) (38,605)
Net unrealized gains included in earnings related to instruments still held at end of period $ 0 $ 0 $ 0 $ 0