CONDENSED CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS’ EQUITY - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]
Balance at January 1, 2019	$ 2,705,714	$ 361	$ 2,339,253	$ (9,020)	$ 375,120
Balance (in shares) at Dec. 31, 2017		36,107,000
Beginning balance at Dec. 31, 2017	2,651,250	$ 361	2,339,253	(9,020)	320,656
Net income	52,231				52,231
Other comprehensive income (loss)	951			951
Exercise of stock options (in shares)		111,000
Exercise of stock options	9,328	$ 1	9,327
Restricted stock grants (in shares)		114,000
Restricted stock grants	0	$ 1	(1)
Restricted stock grants surrendered (in shares)		(47,000)
Restricted stock grants surrendered	(15,392)		(15,392)
Stock-based compensation expense	10,335		10,335
Employee stock purchase plan (in shares)		4,000
Employee stock purchase plan	1,431		1,431
Stock issued for acquisitions (in shares)		103,000
Stock issued for acquisitions	36,366	$ 1	36,365
Balance (in shares) at Mar. 31, 2018		36,392,000
Ending balance at Mar. 31, 2018	2,800,964	$ 364	2,381,318	(8,069)	427,351
Balance (in shares) at Dec. 31, 2017		36,107,000
Beginning balance at Dec. 31, 2017	2,651,250	$ 361	2,339,253	(9,020)	320,656
Net income	96,066
Other comprehensive income (loss)	(1,108)
Balance (in shares) at Jun. 30, 2018		36,391,000
Ending balance at Jun. 30, 2018	2,850,618	$ 364	2,389,196	(10,128)	471,186
Balance (in shares) at Mar. 31, 2018		36,392,000
Beginning balance at Mar. 31, 2018	2,800,964	$ 364	2,381,318	(8,069)	427,351
Net income	43,835				43,835
Other comprehensive income (loss)	(2,059)			(2,059)
Exercise of stock options (in shares)		12,000
Exercise of stock options	2,261		2,261
Restricted stock grants (in shares)		12,000
Restricted stock grants	0		0
Restricted stock grants surrendered (in shares)		(29,000)
Restricted stock grants surrendered	(7,002)		(7,002)
Stock-based compensation expense	11,135		11,135
Employee stock purchase plan (in shares)		4,000
Employee stock purchase plan	1,484		1,484
Balance (in shares) at Jun. 30, 2018		36,391,000
Ending balance at Jun. 30, 2018	2,850,618	$ 364	2,389,196	(10,128)	471,186
Balance at January 1, 2019	3,033,999	$ 364	2,419,812	(11,688)	625,511
Balance (in shares) at Dec. 31, 2018		36,446,000
Beginning balance at Dec. 31, 2018	3,021,942	$ 364	2,419,812	(11,688)	613,454
Net income	85,169				85,169
Other comprehensive income (loss)	380			380
Exercise of stock options (in shares)		79,000
Exercise of stock options	10,638	$ 1	10,637
Restricted stock grants (in shares)		132,000
Restricted stock grants	0	$ 1	(1)
Restricted stock grants surrendered (in shares)		(43,000)
Restricted stock grants surrendered	(18,679)		(18,679)
Stock-based compensation expense	12,034		12,034
Management stock purchase plan	3,491
Employee stock purchase plan (in shares)		4,000
Employee stock purchase plan	1,582	$ 0	1,582
Balance (in shares) at Mar. 31, 2019		36,618,000
Ending balance at Mar. 31, 2019	3,128,614	$ 366	2,428,876	(11,308)	710,680
Balance (in shares) at Dec. 31, 2018		36,446,000
Beginning balance at Dec. 31, 2018	3,021,942	$ 364	2,419,812	(11,688)	613,454
Net income	148,417
Other comprehensive income (loss)	(387)
Balance (in shares) at Jun. 30, 2019		36,624,000
Ending balance at Jun. 30, 2019	3,204,071	$ 366	2,441,852	(12,075)	773,928
Balance (in shares) at Mar. 31, 2019		36,618,000
Beginning balance at Mar. 31, 2019	3,128,614	$ 366	2,428,876	(11,308)	710,680
Net income	63,248				63,248
Other comprehensive income (loss)	(767)			(767)
Exercise of stock options (in shares)		15,000
Exercise of stock options	3,001		3,001
Restricted stock grants (in shares)		4,000
Restricted stock grants	0		0
Restricted stock grants surrendered (in shares)		(17,000)
Restricted stock grants surrendered	(5,546)		(5,546)
Stock-based compensation expense	13,706		13,706
Employee stock purchase plan (in shares)		4,000
Employee stock purchase plan	1,815	$ 0	1,815
Balance (in shares) at Jun. 30, 2019		36,624,000
Ending balance at Jun. 30, 2019	$ 3,204,071	$ 366	$ 2,441,852	$ (12,075)	$ 773,928