Stock Plans and Stock Based Compensation - Net Cash Proceeds, Tax Benefits and Intrinsic Value Related to Stock Options, SARs, and Restricted Stock(Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Net proceeds from stock option exercises	$ (641)	$ (806)	$ 1,382
Tax benefits related to stock option and SAR exercises and vesting of restricted shares	1,634	1,433	358
Intrinsic value of stock option exercises	$ 3,139	$ 3,718	$ 3,014