Income Taxes - Tax refund (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Operating loss carryforwards
Unrecognized tax benefits, income tax penalties accrued	$ 0.0	$ 0.0
Tax Year 2014
Operating loss carryforwards
Federal income tax refund received	21.9
Tax Year 2016
Operating loss carryforwards
Federal income tax refund received	$ 4.4