Unaudited Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities
Net income	$ 119,086	$ 135,196
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	39,073	30,644
Accretion of discount on available-for-sale securities	(1,374)	(646)
Amortization of debt issuance costs	27	26
Stock-based compensation expense	56,531	41,122
Cash paid for derivative settlement	(424)	(26)
Loss on derivative	2,042	1,789
Deferred income taxes, net	11,204	15,715
Changes in operating assets and liabilities:
Accounts receivable	(3,057)	805
Prepaid expenses	(7,196)	(5,066)
Inventory	32	(181)
Other assets	(3,757)	(2,367)
Deferred contract costs	(56,922)	(53,654)
Accounts payable	4,491	(970)
Income taxes, net	(5,260)	(5,550)
Accrued commissions and bonuses	(4,403)	(2,239)
Accrued payroll and vacation	12,853	6,931
Deferred revenue	9,143	8,714
Accrued expenses and other current liabilities	2,239	6,176
Net cash provided by operating activities	174,328	176,419
Cash flows from investing activities
Purchase of short-term investments from funds held for clients	(217,858)	(61,268)
Proceeds from maturities of short-term investments from funds held for clients	93,593	54,200
Purchases of property and equipment	(73,502)	(71,080)
Net cash used in investing activities	(197,767)	(78,148)
Cash flows from financing activities
Repurchases of common stock	(52,040)
Withholding taxes paid related to net share settlements	(26,099)	(42,229)
Payments on long-term debt	(1,331)	(1,331)
Net change in client funds obligation	(140,421)	(131,869)
Payment of debt issuance costs		(16)
Net cash used in financing activities	(219,891)	(175,445)
Decrease in cash, cash equivalents, restricted cash and restricted cash equivalents	(243,330)	(77,174)
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period	1,641,854	986,464
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period	1,398,524	909,290
Cash and cash equivalents	156,398	108,127
Restricted cash included in funds held for clients	1,242,126	801,163
Non-cash investing and financing activities:
Purchases of property and equipment, accrued but not paid	2,707	2,190
Stock-based compensation for capitalized software	5,284	4,287
Right of use assets obtained in exchange for operating lease liabilities	$ 9,323	$ 9,924