Income Taxes - Effective Tax Rate and Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Effective tax rate (as a percent)
Effective Income Tax Rate Reconciliation, Percent	22.30%	34.40%	17.50%	33.00%
Effect of Tax Cuts and Jobs Act of 2017 [Abstract]
Federal corporate income tax rate (as a percent)						35.00%
Uncertain tax positions
Amount of unrecognized tax benefits	$ 24.0		$ 24.0			$ 23.8
Decrease in unrecognized tax benefits			0.2
Gross accrued interest and penalties on unrecognized tax benefits	2.4		2.4			2.0
Unrecognized income tax benefits that, if recognized, would affect the effective tax rates	25.2		25.2			24.5
Unrecognized income tax benefits that, if recognized, would affect the effective tax rates , interest and penalties	1.6		1.6			$ 1.3
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	$ 0.0		$ 0.0
Expected
Effect of Tax Cuts and Jobs Act of 2017 [Abstract]
Federal corporate income tax rate (as a percent)					21.00%