Income Taxes - Reconciliation of Tax Expense at Effective Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Expense, Continuing Operations Income Tax Reconciliation [Abstract]
Tax expense at the statutory tax rate	$ 49.4	$ 43.3	$ 54.9
Increase (decrease) in tax resulting from:
Tax-exempt income	(2.8)	(2.8)	(4.5)
State income tax, net of federal income tax benefit	9.9	8.5	4.9
Benefit of stock-based compensation plans			(2.6)
Federal tax credits	(2.0)	(2.6)	(2.5)
Benefit due to enactment of federal tax reform	0.0	0.0	2.2
Other, net	0.8	0.8	2.2
Total income tax expense	$ 54.1	$ 46.1	$ 50.2