Derivative Instruments - Derivative Assets and Derivative Liabilities (Details) $ in Millions	Dec. 31, 2019 USD ($) Instruments	Dec. 31, 2018 USD ($)
Derivative assets
Other current derivative assets	$ 1.9	$ 5.1
Other long-term derivative assets	0.1	0.0
Derivative assets	2.0	5.1
Derivative liabilities
Other current derivative liabilities	5.3	1.3
Other long-term derivative liabilities	0.1	0.0
Derivative liabilities	5.4	1.3
Natural gas contracts
Derivative assets
Other current derivative assets	0.4	0.7
Other long-term derivative assets	0.0	0.0
Derivative liabilities
Other current derivative liabilities	5.1	1.2
Other long-term derivative liabilities	0.1	0.0
FTRs
Derivative assets
Other current derivative assets	1.5	4.4
Derivative liabilities
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Other current derivative assets	0.0	0.0
Other long-term derivative assets	0.1	0.0
Derivative liabilities
Other current derivative liabilities	0.2	0.1
Other long-term derivative liabilities	$ 0.0	$ 0.0
Derivatives designated as hedging instruments
Derivative instruments
Number of derivative instruments | Instruments	0