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Fair Value Measurements - Net Change in Assets and Liabilities Measured at Fair Value on a Recurring Basis and Included in the Level 3 Fair Value Category (Details) - Commodity - USD ($)
$ in Millions
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]        
Beginning balance $ 3.9 $ (0.2) $ 2.4 $ 0.0
Included in earnings [1] 0.0 0.0 0.1 (0.1)
Included in regulatory assets/liabilities 3.0 0.3 4.5 0.0
Settlements 0.0 0.0 (0.1) 0.1
Purchases 0.0 0.0 0.0 0.1
Ending balance $ 6.9 $ 0.1 $ 6.9 $ 0.1
[1] The gains and losses included in earnings were classified in purchased gas.