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CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Statement of Comprehensive Income [Abstract]        
Net income $ 231.3 $ 218.7 $ 401.5 $ 339.6
Pension Plan and Restoration of Retirement Income Plan:        
Amortization of prior service cost, net of tax of $0.0, $0.0, $0.1 and $0.0, respectively 0.1 0.1 0.1 0.2
Amortization of deferred net loss, net of tax of $0.0, $0.0, $0.0 and $0.1, respectively 0.1 0.0 0.3 0.1
Regulatory classification of certain pension costs, net of tax of $0.0, $0.0, $0.0 and $1.2, respectively 0.0 0.0 0.0 3.9
Postretirement benefit plans:        
Amortization of deferred net gain, net of tax of $(0.0), $(0.0), $(0.1) and $(0.0), respectively (0.1) 0.0 (0.1) (0.1)
Other comprehensive income, net of tax 0.1 0.1 0.3 4.1
Comprehensive income $ 231.4 $ 218.8 $ 401.8 $ 343.7