Income Taxes - Tax cuts and jobs act (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
U.S. Federal statutory tax rate	21.00%	21.00%	35.00%
Write down of deferred tax assets	$ 42.5
Reduction in valuation allowance	$ 42.5
Reduction in tax benefit due to write down of deferred tax assets	33.5
Valuation allowance rate offsetting tax rate reduction	33.50%