Income Taxes - Reconciliation of Tax Expense at Effective Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Expense, Continuing Operations Income Tax Reconciliation [Abstract]
Tax expense at the statutory tax rate	$ 52.0	$ 44.0	$ 49.4
Increase (decrease) in tax resulting from:
Tax-exempt income	(2.8)	(2.1)	(2.8)
State income tax, net of federal income tax benefit	9.9	9.0	9.9
Benefit of stock-based compensation plans			(1.2)
Federal tax credits	(4.3)	(2.3)	(2.0)
Other, net	0.6	(0.1)	0.6
Total income tax expense	$ 55.7	$ 48.1	$ 54.1