Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
CASH FLOWS FROM (USED IN) OPERATING ACTIVITIES:
Net income	$ 137,017,000	$ 120,530,000	$ 113,890,000
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	77,075,000	70,300,000	55,879,000
Noncash lease expense	36,065,000	28,169,000	27,619,000
Deferred income taxes	17,450,000	9,856,000	15,739,000
Stock-based compensation expense	15,369,000	11,366,000	7,493,000
Bad debt expense	(6,022,000)	5,244,000	3,000,000
Contract loss reserve	0	(372,000)	(1,481,000)
Change in assets and liabilities—net of effects from acquired businesses:
Receivables-net	(3,954,000)	1,298,000	24,660,000
Prepaid expenses	(1,790,000)	(5,963,000)	419,000
Taxes receivable—current	1,470,000	28,000	(21,996,000)
Other current assets	818,000	(987,000)	4,060,000
Employee supplemental savings plan asset	(5,764,000)	(5,208,000)	(6,297,000)
Other assets	(4,941,000)	(1,827,000)	97,000
Accounts payable	14,475,000	(303,000)	14,707,000
Accrued salaries and related expenses	(18,040,000)	24,666,000	2,796,000
Contract liabilities	(5,350,000)	9,149,000	(589,000)
Operating lease obligations	(35,675,000)	(31,055,000)	(28,520,000)
Accrued expenses and other current liabilities	(6,625,000)	9,248,000	(3,857,000)
Accrued retirement	(257,000)	758,000	4,553,000
Other long-term liabilities	1,068,000	2,010,000	9,380,000
Other	(218,000)	337,000	(146,000)
Net cash flow from operating activities	212,171,000	247,244,000	221,406,000
CASH FLOWS FROM (USED IN) INVESTING ACTIVITIES:
Acquisition of businesses-net of cash acquired	(370,811,000)	(78,815,000)	(152,851,000)
Purchases of property and equipment	(54,908,000)	(71,129,000)	(54,795,000)
Proceeds from sale of property and equipment	302,000	869,000	0
Proceeds from corporate owned life insurance	270,000	4,137,000	21,000
Investment in capitalized software for internal use	(9,000)	(5,193,000)	(3,677,000)
Deferred contract costs	0	0	(3,878,000)
Proceeds from equity method investment	0	0	283,000
Net cash (used in) investing activities	(425,156,000)	(150,131,000)	(214,897,000)
CASH FLOWS FROM (USED IN) FINANCING ACTIVITIES:
Borrowings under credit facility	507,500,000	302,500,000	624,000,000
Repayments under credit facility	(222,500,000)	(324,000,000)	(595,000,000)
Dividends paid	(61,823,000)	(51,618,000)	(43,205,000)
Proceeds from exercise of stock options	9,758,000	10,249,000	12,895,000
Payment consideration to tax authority on employee's behalf	(4,267,000)	(1,746,000)	(1,503,000)
Debt issuance costs	(3,315,000)	0	0
Principal paid on financing leases	(187,000)	(159,000)	(136,000)
Net cash from (used in) financing activities	225,166,000	(64,774,000)	(2,949,000)
NET CHANGE IN CASH AND CASH EQUIVALENTS	12,181,000	32,339,000	3,560,000
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	41,193,000	8,854,000	5,294,000
CASH AND CASH EQUIVALENTS, END OF PERIOD	53,374,000	41,193,000	8,854,000
SUPPLEMENTAL CASH FLOW INFORMATION
Cash paid for interest	1,769,000	1,819,000	2,436,000
Noncash investing and financing activities:
Noncash investing activities	1,861,000	5,480,000	5,981,000
Operating lease obligations arising from obtaining right of use assets	15,695,000	6,459,000	31,010,000
Finance lease obligations arising from obtaining right of use assets	$ 141,000	$ 107,000	$ 368,000