Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities
Net income	$ 195,960	$ 143,453	$ 180,576
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	67,222	53,373	42,211
Accretion of discount on available-for-sale securities	(452)	(1,563)	(940)
Non-cash marketing expense	1,051
Loss on disposition of property and equipment	146
Amortization of debt issuance costs	36	36	35
Stock-based compensation expense	97,506	90,108	47,268
Cash paid for derivative settlement	(741)	(613)	(81)
(Gain)/loss on derivative	(662)	1,993	1,456
Deferred income taxes, net	32,906	21,381	21,011
Changes in operating assets and liabilities:
Accounts receivable	(360)	168	(5,884)
Prepaid expenses	(5,875)	(4,293)	(5,903)
Inventory	481	(41)	(403)
Other assets	(7,862)	(1,720)	(3,555)
Deferred contract costs	(103,356)	(89,776)	(76,204)
Accounts payable	(660)	1,529	(221)
Income taxes, net	(5,966)	(6,427)	(58)
Accrued commissions and bonuses	8,654	1,360	1,672
Accrued payroll and vacation	9,730	9,659	4,129
Deferred revenue	14,600	10,582	11,593
Accrued expenses and other current liabilities	17,004	(2,002)	7,561
Net cash provided by operating activities	319,362	227,207	224,263
Cash flows from investing activities
Purchases of short-term investments from funds held for clients	(398,819)	(332,756)	(195,811)
Proceeds from maturities of short-term investments from funds held for clients	267,341	308,981	69,200
Purchases of intangible assets	(5,500)
Purchases of property and equipment	(120,692)	(94,102)	(92,934)
Net cash used in investing activities	(257,670)	(117,877)	(219,545)
Cash flows from financing activities
Repurchases of common stock		(52,040)
Withholding taxes paid related to net share settlements	(65,580)	(62,811)	(42,528)
Payments on long-term debt	(1,775)	(1,775)	(1,775)
Net change in client funds obligation	233,079	(49,283)	694,991
Payment of debt issuance costs			(16)
Net cash provided by (used in) financing activities	165,724	(165,909)	650,672
Increase (decrease) in cash, cash equivalents, restricted cash and restricted cash equivalents	227,416	(56,579)	655,390
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	986,464
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period	1,812,691	1,585,275	1,641,854
Cash and cash equivalents	277,978	151,710	133,667
Restricted cash included in funds held for clients	1,534,713	1,433,565	1,508,187
Supplemental disclosures of cash flow information:
Cash paid for interest, net of amounts capitalized	2		891
Cash paid for income taxes	33,068	27,530	24,566
Non-cash investing and financing activities:
Purchases of property and equipment, accrued but not paid	7,581	837	7,451
Stock-based compensation for capitalized software	7,141	6,655	4,757
Right of use assets obtained in exchange for operating lease liabilities	$ 14,141	$ 9,693	$ 14,000