Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities:
Net income	$ 137,148	$ 35,290
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	49,081	42,660
Non-cash interest expense and other	(251)	2,817
Write-off of fees on credit facilities and senior notes	158	0
Change in fair value of contingent consideration	0	5,000
Increase (decrease) in allowance for credit losses and sales credits	(526)	6,742
Provision for deferred income taxes	(1,390)	(9,827)
Share-based compensation	15,306	11,274
Loss on disposal or sale of fixed assets	383	3,321
Amortization of discount on investments	(35)	(71)
Net loss on deferred compensation balances	245	379
Non-cash lease expense	(794)	180
Changes in assets and liabilities, net of effects from acquisitions:
Accounts receivable	(91,911)	2,160
Accounts receivable, subcontractor	(56,424)	14,245
Income taxes receivable	4,791	6,157
Prepaid expenses	(4,186)	(3,260)
Other current assets	3,308	584
Other assets	962	3,144
Accounts payable and accrued expenses	91,823	(14,035)
Accrued compensation and benefits	65,101	(1,187)
Other liabilities	(5,765)	22,764
Deferred revenue	3,582	(72)
Restricted investments balance	19	6
Net cash provided by operating activities	210,625	128,271
Cash flows from investing activities:
Purchase and development of fixed assets	(23,069)	(19,727)
Purchase of investments	(17,995)	(23,129)
Proceeds from maturity of investments	30,700	14,100
Purchase of equity investment	(500)	0
Payments to fund deferred compensation plan	(1,391)	(6,191)
Purchase of convertible promissory notes	0	(490)
Cash paid for acquisitions, net of cash and restricted cash received	(41,264)	(476,491)
Cash paid for other intangibles	(90)	(1,400)
Cash received for working capital adjustments for prior year acquisitions	0	66
Net cash used in investing activities	(53,609)	(513,262)
Cash flows from financing activities:
Payments on term loans	(21,875)	(1,563)
Proceeds from term loans	0	250,000
Payments on revolving credit facility	(70,000)	(125,000)
Proceeds from revolving credit facility	70,000	225,000
Payment of financing costs	0	(4,181)
Earn-out payments to settle contingent consideration liabilities for prior acquisitions	(3,100)	(10,622)
Cash paid for shares withheld for taxes	(5,728)	(4,640)
Net cash provided by (used in) financing activities	(30,703)	328,994
Effect of exchange rate changes on cash	(21)	(105)
Net increase (decrease) in cash, cash equivalents and restricted cash	126,292	(56,102)
Cash, cash equivalents and restricted cash at beginning of period	83,990	153,962
Cash, cash equivalents and restricted cash at end of period	210,282	97,860
Supplemental disclosures of cash flow information:
Cash paid for amounts included in the measurement of operating lease liabilities	9,740	10,112
Cash paid for interest (net of $195 and $208 capitalized for the six months ended June 30, 2021 and 2020, respectively)	19,028	12,759
Cash paid for income taxes	44,061	59
Acquisitions:
Fair value of tangible assets acquired in acquisitions, net of cash and restricted cash received	1,910	35,868
Goodwill	27,726	274,560
Intangible assets	12,440	228,000
Liabilities assumed	(812)	(61,937)
Net cash paid for acquisitions	41,264	476,491
Supplemental disclosures of non-cash investing and financing activities:
Purchase of fixed assets recorded in accounts payable and accrued expenses	$ 3,665	$ 1,225