Derivative Instruments - Derivative assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Derivative Asset [Abstract]
Other current derivative asset	$ 38.2	$ 8.8
Other long-term derivative assets	3.4	1.1
Derivative asset	41.6	9.9
Derivative Liability [Abstract]
Other current derivative liabilities	1.9	49.0
Other long-term derivative Iiabilities	0.5	10.0
Derivative liability	2.4	59.0
Natural gas contracts
Derivative Asset [Abstract]
Other current derivative asset	31.4	2.6
Other long-term derivative assets	2.9	0.5
Derivative Liability [Abstract]
Other current derivative liabilities	0.4	38.5
Other long-term derivative Iiabilities	0.0	3.3
Petroleum products contracts
Derivative Asset [Abstract]
Other current derivative asset	0.2	0.9
Other long-term derivative assets	0.0	0.3
Derivative Liability [Abstract]
Other current derivative liabilities	0.1	3.8
Other long-term derivative Iiabilities	0.0	1.1
FTRs
Derivative Asset [Abstract]
Other current derivative asset	5.1	3.6
Derivative Liability [Abstract]
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative Asset [Abstract]
Other current derivative asset	1.5	1.7
Other long-term derivative assets	0.5	0.3
Derivative Liability [Abstract]
Other current derivative liabilities	1.4	6.7
Other long-term derivative Iiabilities	$ 0.5	$ 5.6