Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect Period Of Adoption Adjustment Member [Member]	Accumulated Other Comprehensive (Loss) Gain [Member]	Accumulated Other Comprehensive (Loss) Gain [Member] Cumulative Effect Period Of Adoption Adjustment Member [Member]
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
Cumulative-effect of accounting changes adjustment at Dec. 31, 2018						$ (101)		$ 101
Net income	71,867				71,867
Other comprehensive loss	(470)						(470)
Share repurchases	(22,826)		$ (22,826)
Share repurchases, Shares			1,144
Restricted stock		$ 2		(2)
Restricted stock, Shares		228
Stock options exercised	4,855	$ 6		4,849
Stock options exercised, Shares		558
Stock-based compensation	5,258			5,258
Business acquisitions	3,949	$ 2		3,947
Business acquisitions, Shares		195
Balance, Amount at Sep. 30, 2019	656,296	$ 1,324	$ (531,356)	706,450	480,729		(851)
Balance, Shares at Sep. 30, 2019		132,385	77,476
Balance, Amount at Jun. 30, 2019	634,060	$ 1,321	$ (530,262)	700,800	462,923		(722)
Balance, Shares at Jun. 30, 2019		132,108	77,428
Net income	17,806				17,806
Other comprehensive loss	(129)						(129)
Share repurchases	(1,094)		$ (1,094)
Share repurchases, Shares			48
Stock options exercised	1,812	$ 2		1,810
Stock options exercised, Shares		180
Stock-based compensation	1,859			1,859
Business acquisitions	1,982	$ 1		1,981
Business acquisitions, Shares		97
Balance, Amount at Sep. 30, 2019	656,296	$ 1,324	$ (531,356)	706,450	480,729		(851)
Balance, Shares at Sep. 30, 2019		132,385	77,476
Balance, Amount at Dec. 31, 2019	659,238	$ 1,331	$ (535,693)	714,704	479,576		(680)
Balance, Shares at Dec. 31, 2019		133,056	77,637
Cumulative-effect of accounting changes adjustment at Dec. 31, 2019	479,576
Net income	78,393				78,393
Other comprehensive loss	(1,815)						(1,815)
Share repurchases	(34,811)		$ (34,811)
Share repurchases, Shares			1,409
Restricted stock, Shares		40
Stock options exercised	$ 4,719	$ 5		4,714
Stock options exercised, Shares	442	442
Stock-based compensation	$ 6,765			6,765
Business acquisitions	5,515	$ 2		5,513
Business acquisitions, Shares		221
Balance, Amount at Sep. 30, 2020	718,004	$ 1,338	$ (570,504)	731,696	557,969		(2,495)
Balance, Shares at Sep. 30, 2020		133,759	79,046
Balance, Amount at Jun. 30, 2020	695,220	$ 1,335	$ (566,762)	725,064	537,892		(2,309)
Balance, Shares at Jun. 30, 2020		133,536	78,881
Net income	20,077				20,077
Other comprehensive loss	(186)						(186)
Share repurchases	(3,742)		$ (3,742)
Share repurchases, Shares			165
Stock options exercised	1,492	$ 2		1,490
Stock options exercised, Shares		115
Stock-based compensation	2,485			2,485
Business acquisitions	2,658	$ 1		2,657
Business acquisitions, Shares		108
Balance, Amount at Sep. 30, 2020	$ 718,004	$ 1,338	$ (570,504)	$ 731,696	$ 557,969		$ (2,495)
Balance, Shares at Sep. 30, 2020		133,759	79,046