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CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Statement of Comprehensive Income [Abstract]    
Net income $ 62.7 $ 18.6
Pension Plan and Restoration of Retirement Income Plan:    
Amortization of prior service cost, net of tax of $0.1 and $0.1, respectively 0.0 0.0
Amortization of deferred net loss, net of tax of $0.0 and $0.0, respectively 0.1 0.1
Regulatory classification of certain pension costs, net of tax of $0.0 and $1.2, respectively 0.0 3.9
Postretirement benefit plans:    
Amortization of deferred net gain, net of tax of $(0.1) and $0.0, respectively 0.0 (0.1)
Other comprehensive income, net of tax 0.1 3.9
Comprehensive income $ 62.8 $ 22.5