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 [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	35,851				35,851
Other comprehensive income	114					114
Share repurchases	(558)		$ (558)
Share repurchases, Shares			31
Stock options exercised	2,271	$ 4		2,267
Stock options exercised, Shares		339
Share-based compensation	1,437			1,437
Share-based compensation, Shares		56
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
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 | ASU 2018-02 [Member]					(101)	101
Net income	37,469				37,469
Other comprehensive income	81					81
Share repurchases	(11,558)		$ (11,558)
Share repurchases, Shares			580
Restricted Stock		$ 1		(1)
Restricted Stock, Shares		173
Stock options exercised	$ 1,400	$ 2		1,398
Stock options exercised, Shares	189	189
Share-based compensation	$ 1,482			1,482
Business acquisitions	950	$ 1		949
Business acquisitions, Shares		47
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