Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets
Derivative assets	$ 2.0	$ 5.1
Liabilities
Derivative liabilities	5.4	1.3
Fair value measurements on a recurring basis
Assets
Derivative assets	2.0	5.1
Liabilities
Derivative liabilities	5.4	1.3
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	0.4	0.7
Liabilities
Derivative liabilities	5.2	1.2
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	0.1	0.0
Liabilities
Derivative liabilities	0.2	0.1
Fair value measurements on a recurring basis | Level 3
Assets
Derivative assets	1.5	4.4
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative assets	0.4	0.7
Liabilities
Derivative liabilities	5.2	1.2
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	0.4	0.7
Liabilities
Derivative liabilities	5.2	1.2
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis | Natural gas contracts | Level 3
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis | FTRs
Assets
Derivative assets	1.5	4.4
Fair value measurements on a recurring basis | FTRs | Level 1
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis | FTRs | Level 2
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis | FTRs | Level 3
Assets
Derivative assets	1.5	4.4
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative assets	0.1
Liabilities
Derivative liabilities	0.2	0.1
Fair value measurements on a recurring basis | Coal contracts | Level 1
Assets
Derivative assets	0.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis | Coal contracts | Level 2
Assets
Derivative assets	0.1
Liabilities
Derivative liabilities	0.2	0.1
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative assets	0.0
Liabilities
Derivative liabilities	$ 0.0	$ 0.0