Derivative Instruments - Derivative assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Derivative Asset
Other current derivative asset	$ 15.3	$ 11.8
Other long-term derivative assets	0.6	0.6
Derivative asset	15.9	12.4
Derivative Liability
Other current derivative liabilities	5.8	10.0
Other long-term derivative Iiabilities	2.1	1.6
Derivative liability	7.9	11.6
Natural gas contracts
Derivative Asset
Other current derivative asset	7.7	5.6
Other long-term derivative assets	0.4	0.1
Derivative Liability
Other current derivative liabilities	5.3	9.4
Other long-term derivative Iiabilities	0.2	1.4
Petroleum products contracts
Derivative Asset
Other current derivative asset	0.0	1.2
Derivative Liability
Other current derivative liabilities	0.0	0.0
FTRs
Derivative Asset
Other current derivative asset	7.4	4.4
Derivative Liability
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative Asset
Other current derivative asset	0.2	0.6
Other long-term derivative assets	0.2	0.5
Derivative Liability
Other current derivative liabilities	0.1	0.6
Other long-term derivative Iiabilities	0.0	0.2
Interest rate swaps
Derivative Asset
Other current derivative asset	0.0	0.0
Other long-term derivative assets	0.0	0.0
Derivative Liability
Other current derivative liabilities	0.4	0.0
Other long-term derivative Iiabilities	$ 1.9	$ 0.0