Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Balance, Amount at Dec. 31, 2015	$ 427,948	$ 1,262	$ (462,167)	$ 634,626	$ 254,860	$ (633)
Balance, Shares at Dec. 31, 2015		126,182	73,228
Net income	40,065				40,065
Other comprehensive income (loss)	129					129
Share repurchases	(9,144)		$ (9,144)
Share repurchases, Shares			919
Restricted stock		$ 3		(3)
Restricted stock, Shares		300
Stock options exercised	8,070	$ 11		8,059
Stock options exercised, Shares		1,128
Share-based compensation	5,725			5,725
Tax benefit (expense) from employee share plans	1,004			1,004
Business acquisitions	6,224	$ 6		6,218
Business acquisitions, Shares		581
Balance, Amount at Dec. 31, 2016	480,021	$ 1,282	$ (471,311)	655,629	294,925	(504)
Balance, Shares at Dec. 31, 2016		128,191	74,147
Net income	50,377				50,377
Other comprehensive income (loss)	322					322
Share repurchases	(19,735)		$ (19,735)
Share repurchases, Shares			1,337
Restricted stock		$ 3		(3)
Restricted stock, Shares		292
Stock options exercised	8,008	$ 12		7,996
Stock options exercised, Shares		1,176
Share-based compensation	5,705			5,705
Business acquisitions	6,181	$ 4		6,177
Business acquisitions, Shares		416
Balance, Amount at Dec. 31, 2017	530,879	$ 1,301	$ (491,046)	675,504	345,302	(182)
Balance, Shares at Dec. 31, 2017		130,075	75,484
Cumulative-effect adjustment (Note 1)	2,091				2,091
Adjusted balance, Amount at January 1, 2018	532,970	$ 1,301	$ (491,046)	675,504	347,393	(182)
Adjusted balance, Shares at January 1, 2018		130,075	75,484
Net income	61,570				61,570
Other comprehensive income (loss)	(300)					(300)
Share repurchases	(17,484)		$ (17,484)
Share repurchases, Shares			848
Restricted stock		$ 3		(3)
Restricted stock, Shares		272
Stock options exercised	$ 6,291	$ 8		6,283
Stock options exercised, Shares	864	864
Share-based compensation	$ 6,866			6,866
Business acquisitions	3,750	$ 2		3,748
Business acquisitions, Shares		193
Balance, Amount at Dec. 31, 2018	$ 593,663	$ 1,314	$ (508,530)	$ 692,398	$ 408,963	$ (482)
Balance, Shares at Dec. 31, 2018		131,404	76,332