Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - Fair value measurements on a recurring basis - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Level 1
Assets
Derivative asset	$ 1.7	$ 0.4
Liabilities
Derivative liability	13.6	6.8
Level 1 | Natural Gas
Assets
Derivative asset	0.5	0.4
Liabilities
Derivative liability	9.2	6.8
Level 1 | FTRs
Assets
Derivative asset	0.0	0.0
Level 1 | Petroleum Products
Assets
Derivative asset	1.2
Liabilities
Derivative liability	4.4
Level 1 | Coal contracts
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Level 2
Assets
Derivative asset	2.0	5.2
Liabilities
Derivative liability	7.8	0.5
Level 2 | Natural Gas
Assets
Derivative asset	0.0	1.9
Liabilities
Derivative liability	0.2	0.3
Level 2 | FTRs
Assets
Derivative asset	0.0	0.0
Level 2 | Petroleum Products
Assets
Derivative asset	0.0
Liabilities
Derivative liability	0.0
Level 2 | Coal contracts
Assets
Derivative asset	2.0	3.3
Liabilities
Derivative liability	7.6	0.2
Level 3
Assets
Derivative asset	1.6	7.0
Liabilities
Derivative liability	0.0	0.0
Level 3 | Natural Gas
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Level 3 | FTRs
Assets
Derivative asset	1.6	7.0
Level 3 | Petroleum Products
Assets
Derivative asset	0.0
Liabilities
Derivative liability	0.0
Level 3 | Coal contracts
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Total
Assets
Derivative asset	5.3	12.6
Liabilities
Derivative liability	21.4	7.3
Total | Natural Gas
Assets
Derivative asset	0.5	2.3
Liabilities
Derivative liability	9.4	7.1
Total | FTRs
Assets
Derivative asset	1.6	7.0
Total | Petroleum Products
Assets
Derivative asset	1.2
Liabilities
Derivative liability	4.4
Total | Coal contracts
Assets
Derivative asset	2.0	3.3
Liabilities
Derivative liability	$ 7.6	$ 0.2