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CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Statement of Comprehensive Income [Abstract]    
Net income $ 18.6 $ 38.3
Pension Plan and Restoration of Retirement Income Plan:    
Amortization of prior service cost, net of tax of $0.1 and $0.0, respectively 0.0 0.1
Amortization of deferred net loss, net of tax of $0.0 and $0.1, respectively 0.1 0.2
Regulatory classification of certain pension costs, net of tax of $1.2 and $0.0, respectively 3.9 0.0
Settlement cost, net of tax of $0.0 and $0.2, respectively 0.0 0.9
Postretirement benefit plans:    
Amortization of deferred net gain, net of tax of $0.0 and $0.0, respectively (0.1) (0.1)
Other comprehensive income, net of tax 3.9 1.1
Comprehensive income $ 22.5 $ 39.4