Income Taxes - Rate reconciliation (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal tax at 21% statutory rate, as a percent	21.00%	21.00%
State and local income taxes, net of federal benefit, as a percent	0.91%	1.68%
Differences in tax effects on foreign income, as a percent	(6.43%)	(10.08%)
Federal tax credits, as a percent	0.59%	4.54%
Uncertain tax positions, interest and penalties, as a percent	0.04%	0.13%
Change in valuation allowance, as a percent	(7.02%)	0.00%
Permanent adjustments, as a percent	0.00%	(8.46%)
Other, as a percent	0.01%	(1.24%)
Effective Income Tax Rate Reconciliation, Percent, Total, as a percent	9.10%	7.57%