Income Taxes - Reconciliation of Tax Expense at Effective Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Expense, Continuing Operations Income Tax Reconciliation [Abstract]
Tax expense at the statutory tax rate	$ 43.3	$ 54.9	$ 50.8
Increase (decrease) in tax resulting from:
Tax-exempt income	(2.8)	(4.5)	(4.4)
State income tax, net of federal income tax benefit	8.5	4.9	4.3
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation			(2.2)
Benefit of stock-based compensation plans			0.0
Federal tax credits	(2.6)	(2.5)	(2.1)
Benefit due to enactment of federal tax reform	0.0	(2.2)	0.0
Other, net	0.8	2.2	1.0
Total income tax expense	$ 46.1	$ 50.2	$ 49.6