Derivative Instruments - Derivative assets and liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Derivative Asset
Other current derivative assets	$ 24.0	$ 8.8
Other long-term derivative assets	1.4	1.1
Derivative asset	25.4	9.9
Derivative Liability
Other current derivative liabilities	4.7	49.0
Other long-term derivative liabilities	1.1	10.0
Derivative liability	5.8	59.0
Natural gas contracts
Derivative Asset
Other current derivative assets	13.2	2.6
Other long-term derivative assets	0.9	0.5
Derivative Liability
Other current derivative liabilities	2.3	38.5
Other long-term derivative liabilities	0.2	3.3
Petroleum products contracts
Derivative Asset
Other current derivative assets	0.3	0.9
Other long-term derivative assets	0.0	0.3
Derivative Liability
Other current derivative liabilities	1.0	3.8
Other long-term derivative liabilities	0.0	1.1
FTRs
Derivative Asset
Other current derivative assets	9.2	3.6
Derivative Liability
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative Asset
Other current derivative assets	1.3	1.7
Other long-term derivative assets	0.5	0.3
Derivative Liability
Other current derivative liabilities	1.4	6.7
Other long-term derivative liabilities	$ 0.9	$ 5.6