Income Taxes - Reconciliation of Tax Expense at Effective Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Expense, Continuing Operations Income Tax Reconciliation [Abstract]
Tax expense at the statutory tax rate	$ 44.0	$ 49.4	$ 43.3
Increase (decrease) in tax resulting from:
Tax-exempt income	(2.1)	(2.8)	(2.8)
State income tax, net of federal income tax benefit	9.0	9.9	8.5
Benefit of stock-based compensation plans			(1.1)
Federal tax credits	(2.3)	(2.0)	(2.6)
Other, net	(0.1)	0.8	0.8
Total income tax expense	$ 48.1	$ 54.1	$ 46.1