Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Effective tax rate (as a percent)	22.90%	21.30%
Statutory Federal income tax rate (as a percent)	21.00%	21.00%
Capital loss carryforward valuation allowance	$ 0.9	$ 0.9
Revaluation of net deferred tax liabilities for the Tax Act			$ 246.8
Alternative minimum tax credits			$ 13.9
Accounting Standards Update 2016-09
Discrete tax benefit		$ 4.3