Employee Benefits - Amounts Not Yet Recognized in Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Pension Benefits
Pre-tax accumulated other comprehensive income (loss)
Net actuarial loss (gain)	$ 12.3	$ 12.7
Prior service credits	0.0	0.0
Total	12.3	12.7
Net regulatory assets (liabilities)
Net actuarial loss (gain)	578.7	688.9
Prior service credits	(2.1)	(2.2)
Total	576.6	686.7
OPEB Benefits
Pre-tax accumulated other comprehensive income (loss)
Net actuarial loss (gain)	(1.1)	(1.2)
Prior service credits	0.0	0.0
Total	(1.1)	(1.2)
Net regulatory assets (liabilities)
Net actuarial loss (gain)	(148.8)	(166.3)
Prior service credits	(15.8)	(29.3)
Total	$ (164.6)	$ (195.6)