Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Assets
Derivative assets	$ 18.0	$ 74.8
Liabilities
Derivative liabilities	106.4	96.6
Fair value measurements on a recurring basis
Assets
Derivative assets	18.0	74.8
Investments held in rabbi trust	51.7	50.9
Liabilities
Derivative liabilities	106.4
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	2.2	16.3
Investments held in rabbi trust	51.7	50.9
Liabilities
Derivative liabilities	70.1
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	8.6	50.7
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	36.3
Fair value measurements on a recurring basis | Level 3
Assets
Derivative assets	7.2	7.8
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	0.0
Fair value measurements on a recurring basis | Natural gas contracts
Assets
Derivative assets	10.5	32.5
Liabilities
Derivative liabilities	86.1	96.6
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	2.2	16.3
Liabilities
Derivative liabilities	70.1	81.4
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	8.3	16.2
Liabilities
Derivative liabilities	16.0	15.2
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 and TCRs
Assets
Derivative assets	7.2	7.8
Fair value measurements on a recurring basis | FTRs and TCRs | Level 1
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis | FTRs and TCRs | Level 2
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis | FTRs and TCRs | Level 3
Assets
Derivative assets	7.2	7.8
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative assets	0.3	34.5
Liabilities
Derivative liabilities	20.3
Fair value measurements on a recurring basis | Coal contracts | Level 1
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities	0.0
Fair value measurements on a recurring basis | Coal contracts | Level 2
Assets
Derivative assets	0.3	34.5
Liabilities
Derivative liabilities	20.3
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative assets	0.0	$ 0.0
Liabilities
Derivative liabilities	$ 0.0