Consolidated Statements of Stockholders' Equity - 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, 2014	$ 399,845	$ 1,188	$ (425,685)	$ 604,284	$ 220,753	$ (695)
Balance, Shares at Dec. 31, 2014		118,820	69,333
Net income	34,107				34,107
Other comprehensive income	62					62
Share repurchases	(36,482)		$ (36,482)
Share repurchases, Shares			3,895
Restricted stock		$ 4		(4)
Restricted stock, Shares		360
Stock options exercised	10,728	$ 15		10,713
Stock options exercised, Shares		1,548
Share-based compensation	5,729			5,729
Tax benefit (expense) from employee share plans	772			772
Convertible bond retirement	9,473	$ 51		9,422
Convertible bond retirement, Shares		5,069
Business acquisitions	3,714	$ 4		3,710
Business acquisitions, Shares		385
Balance, Amount at Dec. 31, 2015	427,948	$ 1,262	$ (462,167)	634,626	254,860	(633)
Balance, Shares at Dec. 31, 2015		126,182	73,228
Net income	40,065				40,065
Other comprehensive income	129					129
Share repurchases	(9,144)		$ (9,144)
Share repurchases, Shares			919
Restricted stock		$ 3		(3)
Restricted stock, Shares		300
Stock options exercised	8,070	$ 11		8,059
Stock options exercised, Shares		1,128
Share-based compensation	5,725			5,725
Tax benefit (expense) from employee share plans	1,004			1,004
Business acquisitions	6,224	$ 6		6,218
Business acquisitions, Shares		581
Balance, Amount at Dec. 31, 2016	480,021	$ 1,282	$ (471,311)	655,629	294,925	(504)
Balance, Shares at Dec. 31, 2016		128,191	74,147
Net income	50,377				50,377
Other comprehensive income	322					322
Share repurchases	(19,735)		$ (19,735)
Share repurchases, Shares			1,337
Restricted stock		$ 3		(3)
Restricted stock, Shares		292
Stock options exercised	$ 8,008	$ 12		7,996
Stock options exercised, Shares	1,176	1,176
Share-based compensation	$ 5,705			5,705
Business acquisitions	6,181	$ 4		6,177
Business acquisitions, Shares		416
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