Shareholders' Equity - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 05, 2017	Dec. 31, 2014
Class of Stock [Line Items]
Stockholders' equity	$ 401,632	$ 362,647	$ 301,459		$ 264,951
Increase in shareholders' equity		39,000
Net income	39,796	38,766	39,135
Common stock issued under various employee plans, net	12,246	10,049	6,342
Cash dividends paid	$ (12,748)	$ (12,187)	$ (8,630)
Shares available to be purchased (in shares)			1,000,000
Common stock, shares issued (in shares)		1,000,000
Shelf registration, maximum amount				$ 100,000
Antidilutive securities excluded from computation of earnings per share (shares)	543,000	456,666	170,000
Common Stock
Class of Stock [Line Items]
Stockholders' equity	$ 217,555	$ 205,309	$ 169,848		162,575
Common stock issued under various employee plans, net	12,246	10,049	6,342
Retained Earnings
Class of Stock [Line Items]
Stockholders' equity	183,694	156,646	130,067		$ 99,562
Net income	39,796	38,766	39,135
Cash dividends paid	$ (12,748)	(12,187)	$ (8,630)
Cash Exercise | Warrant
Class of Stock [Line Items]
Aggregate intrinsic value		$ 2,700