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
		Mar. 31, 2018	Mar. 31, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance		$ 2.4	$ 0.0
Included in earnings	[1]	0.1	(0.1)
Included in regulatory assets/liabilities		(1.0)	(1.3)
Settlements		(0.1)	0.1
Ending balance		$ 1.4	$ (1.3)

[1] The gains and losses included in earnings were classified in purchased gas.