Derivative Instruments - Derivative assets and liabilities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Derivative Asset
Other current derivative assets	$ 12.9		$ 12.9		$ 14.7
Other long-term derivative assets	0.6		0.6		0.6
Derivative asset	13.5		13.5		15.3
Derivative Liability
Other current derivative liabilities	39.9		39.9		11.7
Other long-term derivative liabilities	6.6		6.6		0.8
Derivative liability	46.5		46.5		12.5
Notional volumes and gains and losses
Gains (losses)	(11.0)	$ 1.2	(21.1)	$ 21.5
Natural Gas
Derivative Asset
Other current derivative assets	6.1		6.1		5.0
Other long-term derivative assets	0.4		0.4		0.0
Derivative Liability
Other current derivative liabilities	33.0		33.0		11.5
Other long-term derivative liabilities	$ 3.5		$ 3.5		0.8
Notional volumes and gains and losses
Volume of derivative instruments	24.2 Dth	6.3 Dth	47.5 Dth	31.1 Dth
Gains (losses)	$ (13.2)	$ (0.8)	$ (26.2)	$ 9.3
Petroleum products
Derivative Asset
Other current derivative assets	0.2		0.2		0.0
Other long-term derivative assets	0.2		0.2		0.0
Derivative Liability
Other current derivative liabilities	1.7		1.7		0.0
Other long-term derivative liabilities	$ 0.4		$ 0.4		0.0
Notional volumes and gains and losses
Volume of derivative instruments	2.8 gallons	2.6 gallons	4.5 gallons	7.0 gallons
Gains (losses)	$ (0.9)	$ 0.0	$ (0.9)	$ 0.6
FTRs
Derivative Asset
Other current derivative assets	5.8		5.8		7.0
Derivative Liability
Other current derivative liabilities	$ 0.0		$ 0.0		0.0
Notional volumes and gains and losses
Volume of derivative instruments	8.6 MWh	6.6 MWh	20.7 MWh	19.7 MWh
Gains (losses)	$ 3.1	$ 2.0	$ 6.0	$ 11.6
Coal
Derivative Asset
Other current derivative assets	0.8		0.8		2.7
Other long-term derivative assets	0.0		0.0		0.6
Derivative Liability
Other current derivative liabilities	5.2		5.2		0.2
Other long-term derivative liabilities	$ 2.7		$ 2.7		$ 0.0