Fair Value Measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Assets
Derivative asset	$ 12.0	$ 5.3
Liabilities
Derivative liability	0.7	21.4
Fair value measurements on a recurring basis
Assets
Derivative asset	12.0	5.3
Liabilities
Derivative liability	0.7	21.4
Natural gas contracts | Fair value measurements on a recurring basis
Assets
Derivative asset	6.8	0.5
Liabilities
Derivative liability	0.1	9.4
Petroleum products contracts | Fair value measurements on a recurring basis
Assets
Derivative asset	0.2	1.2
Liabilities
Derivative liability	0.1	4.4
FTRs | Fair value measurements on a recurring basis
Assets
Derivative asset	3.1	1.6
Coal contracts | Fair value measurements on a recurring basis
Assets
Derivative asset	1.9	2.0
Liabilities
Derivative liability	0.5	7.6
Level 1 | Fair value measurements on a recurring basis
Assets
Derivative asset	6.2	1.7
Liabilities
Derivative liability	0.2	13.6
Level 1 | Natural gas contracts | Fair value measurements on a recurring basis
Assets
Derivative asset	6.0	0.5
Liabilities
Derivative liability	0.1	9.2
Level 1 | Petroleum products contracts | Fair value measurements on a recurring basis
Assets
Derivative asset	0.2	1.2
Liabilities
Derivative liability	0.1	4.4
Level 1 | FTRs | Fair value measurements on a recurring basis
Assets
Derivative asset	0.0	0.0
Level 1 | Coal contracts | Fair value measurements on a recurring basis
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Level 2 | Fair value measurements on a recurring basis
Assets
Derivative asset	2.7	2.0
Liabilities
Derivative liability	0.5	7.8
Level 2 | Natural gas contracts | Fair value measurements on a recurring basis
Assets
Derivative asset	0.8	0.0
Liabilities
Derivative liability	0.0	0.2
Level 2 | Petroleum products contracts | Fair value measurements on a recurring basis
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Level 2 | FTRs | Fair value measurements on a recurring basis
Assets
Derivative asset	0.0	0.0
Level 2 | Coal contracts | Fair value measurements on a recurring basis
Assets
Derivative asset	1.9	2.0
Liabilities
Derivative liability	0.5	7.6
Level 3 | Fair value measurements on a recurring basis
Assets
Derivative asset	3.1	1.6
Liabilities
Derivative liability	0.0	0.0
Level 3 | Natural gas contracts | Fair value measurements on a recurring basis
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Level 3 | Petroleum products contracts | Fair value measurements on a recurring basis
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Level 3 | FTRs | Fair value measurements on a recurring basis
Assets
Derivative asset	3.1	1.6
Level 3 | Coal contracts | Fair value measurements on a recurring basis
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	$ 0.0	$ 0.0