Income Taxes - Reconciliation of Tax Expense at Effective Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Expense, Continuing Operations Income Tax Reconciliation [Abstract]
Tax expense at the statutory tax rate	$ 70.7	$ 54.0	$ 52.0
Increase (decrease) in tax resulting from:
Tax-exempt income	(8.1)	(8.4)	(2.8)
State income tax, net of federal income tax benefit	13.6	10.6	9.9
Deficiency (benefit) of stock-based compensation plans	0.3	0.2	(0.5)
Federal tax credits	(0.2)	(4.3)	(4.3)
Other, net	1.0	(0.8)	0.6
Total income tax expense	$ 79.3	$ 54.9	$ 55.7