Equity Compensation Plans - 1996 Employee Stock Purchase Plan (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Value of stock purchases made pursuant to employee stock purchase plan	$ 1,547	$ 1,547	$ 1,394
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	0.85%
Issuance of common stock pursuant to employee stock purchase plan, shares	43,655	58,927	61,592
Value of stock purchases made pursuant to employee stock purchase plan	$ 1,600	$ 1,600	$ 1,400
Remaining number of shares eligible for purchase under employee stock purchase plan	396,967
1996 Employee Stock Purchase Plan | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase price of shares under the plan	$ 29.70	$ 20.84	$ 21.31
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	$ 30.58	$ 25.47