Unaudited Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities
Net income	$ 147,273	$ 119,086
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	48,254	39,073
Accretion of discount on available-for-sale securities	(276)	(1,374)
Non-cash marketing expense	618
Loss on disposition of property and equipment	146
Amortization of debt issuance costs	27	27
Stock-based compensation expense	76,364	56,531
Cash paid for derivative settlement	(558)	(424)
(Gain)/loss on derivative	(305)	2,042
Deferred income taxes, net	12,630	11,204
Changes in operating assets and liabilities:
Accounts receivable	(3,874)	(3,057)
Prepaid expenses	(8,789)	(7,196)
Inventory	97	32
Other assets	671	(3,757)
Deferred contract costs	(68,041)	(56,922)
Accounts payable	1,659	4,491
Income taxes, net	236	(5,260)
Accrued commissions and bonuses	(1,272)	(4,403)
Accrued payroll and vacation	11,609	12,853
Deferred revenue	11,051	9,143
Accrued expenses and other current liabilities	2,117	2,239
Net cash provided by operating activities	229,637	174,328
Cash flows from investing activities
Purchase of short-term investments from funds held for clients	(170,760)	(217,858)
Proceeds from maturities of short-term investments from funds held for clients	195,000	93,593
Purchases of intangible assets	(1,500)
Purchases of property and equipment	(86,718)	(73,502)
Net cash used in investing activities	(63,978)	(197,767)
Cash flows from financing activities
Repurchases of common stock		(52,040)
Withholding taxes paid related to net share settlements	(61,149)	(26,099)
Payments on long-term debt	(1,331)	(1,331)
Net change in client funds obligation	1,346,479	(140,421)
Net cash provided by (used in) financing activities	1,283,999	(219,891)
Increase (decrease) in cash, cash equivalents, restricted cash and restricted cash equivalents	1,449,658	(243,330)
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period	1,585,275	1,641,854
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period	3,034,933	1,398,524
Cash and cash equivalents	230,926	156,398
Restricted cash included in funds held for clients	2,804,007	1,242,126
Non-cash investing and financing activities:
Purchases of property and equipment, accrued but not paid	5,994	2,707
Stock-based compensation for capitalized software	5,108	5,284
Right of use assets obtained in exchange for operating lease liabilities	$ 4,439	$ 9,323