Derivative Instruments - Gains (Losses) and Notional Volumes (Details) gal in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) MMBTU MWh gal	Dec. 31, 2018 USD ($) MMBTU MWh gal	Dec. 31, 2017 USD ($) MMBTU MWh gal
Realized gains (losses) on derivatives
Gains (losses)	$ (2.6)	$ 14.3	$ 5.2
Natural gas contracts
Realized gains (losses) on derivatives
Gains (losses)	$ (11.3)	$ 9.7	$ (1.0)
Notional sales volumes
Notional sales volumes | MMBTU	61.6	53.4	26.9
Petroleum products contracts
Realized gains (losses) on derivatives
Gains (losses)	$ 0.0	$ 1.2	$ (1.4)
Notional sales volumes
Notional sales volumes (gallons) | gal	0.0	4.2	16.7
FTRs
Realized gains (losses) on derivatives
Gains (losses)	$ 8.7	$ 3.4	$ 7.6
Notional sales volumes
Notional sales volumes | MWh	21.7	21.2	27.1