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	48,957				48,957
Other comprehensive income	288					288
Share repurchases	(4,409)		$ (4,409)
Share repurchases, Shares			219
Restricted stock		$ 4		(4)
Restricted stock, Shares		272
Stock options exercised	4,425	$ 5		4,420
Stock options exercised, Shares		616
Share-based compensation	3,850			3,850
Business acquisitions	3,214	$ 1		3,213
Business acquisitions, Shares		169
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
Balance, Amount at Mar. 31, 2018	571,616	$ 1,305	$ (491,604)	679,208	382,775	(68)
Balance, Shares at Mar. 31, 2018		130,470	75,515
Net income	13,106				13,106
Other comprehensive income	174					174
Share repurchases	(3,851)		$ (3,851)
Share repurchases, Shares			188
Restricted stock		$ 4		(4)
Restricted stock, Shares		216
Stock options exercised	2,154	$ 1		2,153
Stock options exercised, Shares		277
Share-based compensation	2,413			2,413
Business acquisitions	3,214	$ 1		3,213
Business acquisitions, Shares		169
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
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	54,061				54,061
Other comprehensive income	(341)					(341)
Share repurchases	(21,732)		$ (21,732)
Share repurchases, Shares			1,096
Restricted stock		$ 2		(2)
Restricted stock, Shares		228
Stock options exercised	$ 3,043	$ 4		3,039
Stock options exercised, Shares	378	378
Share-based compensation	$ 3,399			3,399
Business acquisitions	1,967	$ 1		1,966
Business acquisitions, Shares		98
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
Balance, Amount at Mar. 31, 2019	623,487	$ 1,318	$ (520,088)	696,226	446,331	(300)
Balance, Shares at Mar. 31, 2019		131,813	76,912
Net income	16,592				16,592
Other comprehensive income	(422)					(422)
Share repurchases	(10,174)		$ (10,174)
Share repurchases, Shares			516
Restricted stock		$ 1		(1)
Restricted stock, Shares		55
Stock options exercised	1,643	$ 2		1,641
Stock options exercised, Shares		189
Share-based compensation	1,917			1,917
Business acquisitions	1,017			1,017
Business acquisitions, Shares		51
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