CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CONSOLIDATED STATEMENTS OF OPERATIONS
REVENUE	$ 2,856,659	$ 2,615,277	$ 2,182,879
COST OF SERVICES	2,309,676	2,113,334	1,736,600
Gross profit	546,983	501,943	446,279
SELLING, GENERAL AND ADMINISTRATIVE EXPENSES	357,777	340,005	296,986
GAIN ON SALE OF ASSETS	(1,445)	(1,701)	(945)
Operating income	190,651	163,639	150,238
OTHER INCOME (EXPENSE):
Interest income	103	224	73
Interest expense	(8,385)	(9,317)	(3,710)
Changes in the fair value of contingent earn-out obligations	9,119	(2,991)	(2,066)
Other	52	187	4,141
Other income (expense)	889	(11,897)	(1,562)
INCOME BEFORE INCOME TAXES	191,540	151,742	148,676
PROVISION FOR INCOME TAXES	41,401	37,418	35,773
NET INCOME	$ 150,139	$ 114,324	$ 112,903
INCOME PER SHARE:
Basic (in shares)	$ 4.11	$ 3.10	$ 3.03
Diluted (in shares)	$ 4.09	$ 3.08	$ 3.00
SHARES USED IN COMPUTING INCOME PER SHARE:
Basic (in shares)	36,542	36,854	37,202
Diluted (in shares)	36,738	37,131	37,592
DIVIDENDS PER SHARE (in dollars per share)	$ 0.425	$ 0.395	$ 0.330