CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 102.3	$ 88.4	$ 120.9	$ 126.7
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	0.0	0.0	0.1	0.1
Amortization of deferred net loss, net of tax of $0.0, $0.0, $0.1 and $0.1, respectively	0.1	0.3	0.1	0.5
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, $0.1, $0.0 and $0.3, respectively	0.0	0.2	0.0	1.1
Postretirement benefit plans:
Amortization of deferred net gain, net of tax of $0.0, $0.0, $0.0 and $0.0, respectively	0.0	0.0	(0.1)	(0.1)
Other comprehensive income, net of tax	0.1	0.5	4.0	1.6
Comprehensive income	$ 102.4	$ 88.9	$ 124.9	$ 128.3