Equity Compensation Plans - 1996 Employee Stock Purchase Plan (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Value of stock purchases made pursuant to employee stock purchase plan	$ 2,523	$ 2,227	$ 2,311
1996 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized for issue to employees under the employee stock purchase plan			1,700,000
Purchase Price of shares as a percentage of market value			85.00%
Number of shares purchased under employee stock purchase plan	68,552	54,949	68,902
Value of stock purchases made pursuant to employee stock purchase plan	$ 2,500	$ 2,300	$ 2,400
Remaining number of shares eligible for purchase under employee stock purchase plan	153,708
1996 Employee Stock Purchase Plan | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase price of shares under the plan	$ 32.20	$ 30.76	$ 27.00
1996 Employee Stock Purchase Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase price of shares under the plan	$ 42.35	$ 48.99	$ 39.68