Income Taxes - Effective tax rate (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
Federal tax expense at statutory rate	$ 4,019	$ 2,504	$ 2,462
State and local income taxes, net of federal income tax benefit	1,157	608	200
Incentive stock options	48	33	117
Stock-based compensation excess tax benefit	(23)	(60)	(85)
Change to deferred tax as a result of tax reform	0	0	683
Research and development tax credits	(129)
Other	(77)	105	13
Income tax expense	$ 4,995	$ 3,190	$ 3,390

Amount of difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operation, attributable to excess tax benefit for share-based compensation cost.

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