Consolidated Statements of Cash Flows (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities
Net income	$ 47,282	$ 41,160
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	9,347	6,069
Accretion of discounts on available-for-sale securities	(165)	(177)
Amortization of debt issuance costs	9	6
Stock-based compensation expense	31,071	23,222
Cash paid for derivative settlement	(1)	(79)
(Gain)/loss on derivative	540	(738)
Deferred income taxes, net	3,722	4,272
Changes in operating assets and liabilities:
Accounts receivable	(421)	(774)
Prepaid expenses	(1,498)	(2,019)
Inventory	(11)	(193)
Other assets	(702)	(565)
Deferred contract costs	(23,414)	(17,712)
Accounts payable	(1,251)	(710)
Income taxes, net	7,488	4,013
Accrued commissions and bonuses	(6,118)	(6,980)
Accrued payroll and vacation	3,765	4,307
Deferred revenue	2,860	2,153
Accrued expenses and other current liabilities	7,923	2,232
Net cash provided by operating activities	80,426	57,487
Cash flows from investing activities
Purchase of short-term investments from funds held for clients	(16,800)	(67,632)
Proceeds from maturities of short-term investments from funds held for clients	14,500	22,500
Net change in funds held for clients	(435,213)	39,350
Purchases of property and equipment	(14,889)	(18,708)
Net cash used in investing activities	(452,402)	(24,490)
Cash flows from financing activities
Repurchases of common stock		(4,999)
Withholding taxes paid related to net share settlement	(19,669)	(11,871)
Payments on long-term debt	(444)
Net change in client funds obligation	437,678	5,959
Payment of debt issuance costs		(42)
Net cash provided by (used in) financing activities	417,565	(10,953)
Increase in cash and cash equivalents	45,589	22,044
Cash and cash equivalents
Beginning of period	45,718	46,077
End of period	$ 91,307	$ 68,121