Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Income Taxes
Corporate income tax rate as a percentage	21.00%	35.00%
Unrecognized tax benefit from net positive reduction in income tax expense	$ 1.9
Unrecognized tax benefits	5.6	$ 7.7
Netted against fully reserved	4.1
Accrued interest expense related to tax reserves, net of related tax effects	$ 1.2	$ 1.6