Equity Compensation Plans - 1996 Employee Stock Purchase Plan (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Value of stock purchases made pursuant to employee stock purchase plan	$ 1,776	$ 1,547	$ 1,547
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%
Issuance of common stock pursuant to employee stock purchase plan, shares	50,674	43,655	58,927
Value of stock purchases made pursuant to employee stock purchase plan	$ 1,800	$ 1,600	$ 1,600
Remaining number of shares eligible for purchase under employee stock purchase plan	346,293
1996 Employee Stock Purchase Plan | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase price of shares under the plan	$ 31.88	$ 29.70	$ 20.84
1996 Employee Stock Purchase Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase price of shares under the plan	$ 41.14	$ 41.14	$ 30.58