Income Taxes - Reconciliation of Amount Computed by Applying the Federal Statutory Income Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Federal statutory tax on earnings before income taxes	$ 35.9	$ 51.6	$ 40.4
State income taxes, net of federal income tax benefit	2.5	4.0	2.0
Non-deductible officer's compensation	4.7	2.3	2.0
Change in enacted tax rates	(57.7)	0.1	0.7
Windfall deduction from equity compensation	(5.2)	(4.9)	0.0
Other	(0.1)	(2.4)	(0.5)
Income tax expense (benefit)	$ (19.9)	$ 50.7	$ 44.6