Income Taxes - Tax Reform (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2018
Income Taxes
Statutory U.S tax rate	21.00%		35.00%
Charge to income resulting from revaluation of deferred tax assets and liabilities due to Tax Reform			$ 25,108
Deferred tax asset due to Tax Reform		$ 6,104		$ 6,954
Period over which transitional deferred tax liability related to tax reform will be amortized		8 years
Deferred tax liability due to Tax Reform		$ 4,578		$ 6,085