FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Assets
Derivative assets	$ 45.1	$ 26.3
Liabilities
Derivative liabilities	41.9	32.5
Fair value measurements on a recurring basis
Assets
Derivative assets	45.1	26.3
Investments held in rabbi trust	43.4	42.0
Liabilities
Derivative liabilities		32.5
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative assets	1.4	1.5
Investments held in rabbi trust	43.4	42.0
Liabilities
Derivative liabilities		23.3
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative assets	19.4	18.3
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities		8.4
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative assets	24.3	6.5
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities		0.8
Fair value measurements on a recurring basis \| Natural gas contracts
Assets
Derivative assets	20.8	19.8
Liabilities
Derivative liabilities	41.9	31.7
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative assets	1.4	1.5
Liabilities
Derivative liabilities	35.0	23.3
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative assets	19.4	18.3
Liabilities
Derivative liabilities	6.9	8.4
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	24.3	6.5
Liabilities
Derivative liabilities		0.8
Fair value measurements on a recurring basis \| FTRs \| Level 1
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities		0.0
Fair value measurements on a recurring basis \| FTRs \| Level 2
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities		0.0
Fair value measurements on a recurring basis \| FTRs \| Level 3
Assets
Derivative assets	$ 24.3	6.5
Liabilities
Derivative liabilities		$ 0.8