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]	Accumulated Other Comprehensive (Loss) Gain [Member]
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 of accounting changes adjustment	1,622				1,622
Net income	62,543				62,543
Other comprehensive income	343					343
Share repurchases	(8,121)		$ (8,121)
Share repurchases, Shares			379
Restricted stock		$ 3		(3)
Restricted stock, Shares		272
Stock options exercised	6,076	$ 8		6,068
Stock options exercised, Shares		840
Share-based compensation	5,358			5,358
Business acquisitions	3,215	$ 2		3,213
Business acquisitions, Shares		169
Balance, Amount at Sep. 30, 2018	601,915	$ 1,314	$ (499,167)	690,140	409,467	161
Balance, Shares at Sep. 30, 2018		131,356	75,863
Balance, Amount at Jun. 30, 2018	588,826	$ 1,311	$ (495,455)	686,983	395,881	106
Balance, Shares at Jun. 30, 2018		131,132	75,703
Net income	13,586				13,586
Other comprehensive income	55					55
Share repurchases	(3,712)		$ (3,712)
Share repurchases, Shares			160
Restricted stock		$ (1)		1
Stock options exercised	1,651	$ 3		1,648
Stock options exercised, Shares		224
Share-based compensation	1,508			1,508
Business acquisitions	1	$ 1
Balance, Amount at Sep. 30, 2018	601,915	$ 1,314	$ (499,167)	690,140	409,467	161
Balance, Shares at Sep. 30, 2018		131,356	75,863
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					(101)	101
Net income	71,867				71,867
Other comprehensive income	(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	558
Share-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 income	(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
Share-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