Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Treasury Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning balance (in shares) at Dec. 31, 2019		133,056	77,637
Beginning balance at Dec. 31, 2019	$ 659,238	$ 1,331	$ (535,693)	$ 714,704	$ 479,576	$ (680)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	78,393				78,393
Other comprehensive income (loss)	(1,815)					(1,815)
Share repurchases (in shares)			1,312
Share repurchases	(32,771)		$ (32,771)
Indirect repurchase of shares for minimum tax withholding (in shares)			97
Indirect repurchase of shares for minimum tax withholding	(2,040)		$ (2,040)
Restricted stock units and awards (in shares)		40
Stock options exercised (in shares)		442
Stock options exercised	4,719	$ 5		4,714
Stock-based compensation	6,765			6,765
Business acquisitions (in shares)		221
Business acquisitions	5,515	$ 2		5,513
Ending balance (in shares) at Sep. 30, 2020		133,759	79,046
Ending balance at Sep. 30, 2020	718,004	$ 1,338	$ (570,504)	731,696	557,969	(2,495)
Beginning balance (in shares) at Jun. 30, 2020		133,536	78,881
Beginning balance at Jun. 30, 2020	695,220	$ 1,335	$ (566,762)	725,064	537,892	(2,309)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,077				20,077
Other comprehensive income (loss)	(186)					(186)
Share repurchases (in shares)			165
Share repurchases	(3,742)		$ (3,742)
Stock options exercised (in shares)		115
Stock options exercised	1,492	$ 2		1,490
Stock-based compensation	2,485			2,485
Business acquisitions (in shares)		108
Business acquisitions	2,658	$ 1		2,657
Ending balance (in shares) at Sep. 30, 2020		133,759	79,046
Ending balance at Sep. 30, 2020	718,004	$ 1,338	$ (570,504)	731,696	557,969	(2,495)
Beginning balance (in shares) at Dec. 31, 2020		134,144	80,045
Beginning balance at Dec. 31, 2020	702,620	$ 1,341	$ (595,297)	740,970	557,875	(2,269)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	80,507				80,507
Other comprehensive income (loss)	1,071					1,071
Share repurchases (in shares)			2,686
Share repurchases	(84,883)		$ (84,883)
Indirect repurchase of shares for minimum tax withholding (in shares)			92
Indirect repurchase of shares for minimum tax withholding	(3,037)		$ (3,037)
Restricted stock units and awards (in shares)		80
Restricted stock units and awards	$ 0	$ 1		(1)
Stock options exercised (in shares)	635	635
Stock options exercised	$ 7,143	$ 6		7,137
Stock-based compensation	8,359			8,359
Business acquisitions (in shares)		274
Business acquisitions	8,867	$ 3		8,864
Ending balance (in shares) at Sep. 30, 2021		135,133	82,823
Ending balance at Sep. 30, 2021	720,647	$ 1,351	$ (683,217)	765,329	638,382	(1,198)
Beginning balance (in shares) at Jun. 30, 2021		134,892	82,173
Beginning balance at Jun. 30, 2021	712,332	$ 1,349	$ (661,772)	757,421	616,691	(1,357)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	21,691				21,691
Other comprehensive income (loss)	159					159
Share repurchases (in shares)			650
Share repurchases	(21,445)		$ (21,445)
Stock options exercised (in shares)		142
Stock options exercised	1,695	$ 1		1,694
Stock-based compensation	2,905			2,905
Business acquisitions (in shares)		99
Business acquisitions	3,310	$ 1		3,309
Ending balance (in shares) at Sep. 30, 2021		135,133	82,823
Ending balance at Sep. 30, 2021	$ 720,647	$ 1,351	$ (683,217)	$ 765,329	$ 638,382	$ (1,198)