Derivative Instruments - Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivative assets
Other current derivative assets	$ 6.7	$ 15.3
Other long-term derivative assets	0.2	0.6
Derivative assets	6.9	15.9
Derivative liabilities
Other current derivative liabilities	24.8	5.8
Other long-term derivative Iiabilities	4.1	2.1
Derivative liabilities	28.9	7.9
Natural gas contracts
Derivative assets
Other current derivative assets	3.4	7.7
Other long-term derivative assets	0.0	0.4
Derivative liabilities
Other current derivative liabilities	21.8	5.3
Other long-term derivative Iiabilities	0.9	0.2
FTRs
Derivative assets
Other current derivative assets	3.1	7.4
Derivative liabilities
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Other current derivative assets	0.2	0.2
Other long-term derivative assets	0.2	0.2
Derivative liabilities
Other current derivative liabilities	0.2	0.1
Other long-term derivative Iiabilities	0.0	0.0
Interest rate swaps
Derivative assets
Other current derivative assets	0.0	0.0
Other long-term derivative assets	0.0	0.0
Derivative liabilities
Other current derivative liabilities	2.8	0.4
Other long-term derivative Iiabilities	$ 3.2	$ 1.9