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CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Statement of Comprehensive Income [Abstract]        
Net income $ 218.7 $ 241.9 $ 339.6 $ 368.6
Pension Plan and Restoration of Retirement Income Plan:        
Amortization of prior service cost, net of tax of $0.0, $0.0, $0.0 and $0.0, respectively 0.1 0.1 0.2 0.2
Amortization of deferred net loss, net of tax of $0.0, $0.1, $0.1 and $0.2, respectively 0.0 0.1 0.1 0.6
Regulatory classification of certain pension costs, net of tax of $0.0, $0.0, $1.2 and $0.0, respectively 0.0 0.0 3.9 0.0
Settlement cost, net of tax of $0.0, $0.0, $0.0 and $0.3, respectively 0.0 0.0 0.0 1.1
Postretirement benefit plans:        
Amortization of deferred net gain, net of tax of ($0.0), ($0.1), ($0.0) and ($0.1), respectively 0.0 0.0 (0.1) (0.1)
Other comprehensive income, net of tax 0.1 0.2 4.1 1.8
Comprehensive income $ 218.8 $ 242.1 $ 343.7 $ 370.4