Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Assets
Derivative assets	$ 41.1	$ 18.0
Liabilities
Derivative liabilities	13.9	106.4
Fair value measurements on a recurring basis
Assets
Derivative assets	41.1	18.0
Investments held in rabbi trust	52.1	51.7
Liabilities
Derivative liabilities		106.4
Fair value measurements on a recurring basis | Level 1
Assets
Derivative assets	19.6	2.2
Investments held in rabbi trust	52.1	51.7
Liabilities
Derivative liabilities		70.1
Fair value measurements on a recurring basis | Level 2
Assets
Derivative assets	13.7	8.6
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	7.8	7.2
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	33.3	10.5
Liabilities
Derivative liabilities	13.9	86.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Derivative assets	19.6	2.2
Liabilities
Derivative liabilities	7.1	70.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Derivative assets	13.7	8.3
Liabilities
Derivative liabilities	6.8	16.0
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 and TCRs
Assets
Derivative assets	7.8	7.2
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	$ 7.8	7.2
Fair value measurements on a recurring basis | Coal contracts
Assets
Derivative assets		0.3
Liabilities
Derivative liabilities		20.3
Fair value measurements on a recurring basis | Coal contracts | Level 1
Assets
Derivative assets		0.0
Liabilities
Derivative liabilities		0.0
Fair value measurements on a recurring basis | Coal contracts | Level 2
Assets
Derivative assets		0.3
Liabilities
Derivative liabilities		20.3
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Derivative assets		0.0
Liabilities
Derivative liabilities		$ 0.0