Consolidated Statements of Cash Flows (unaudited) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net income		$ 135,196	$ 105,651	[1]
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		30,644	20,910	[1]
Accretion of discounts on available-for-sale securities		(646)	(860)	[1]
Amortization of debt issuance costs		26	24	[1]
Stock-based compensation expense		41,122	31,508	[1]
Cash paid for derivative settlement		(26)	(168)	[1]
(Gain)/loss on derivative		1,789	(1,259)	[1]
Deferred income taxes, net		15,715	15,077	[1]
Changes in operating assets and liabilities:
Accounts receivable		805	(637)	[1]
Prepaid expenses		(5,066)	(2,952)	[1]
Inventory		(181)	(56)	[1]
Other assets		(2,367)	(111)	[1]
Deferred contract costs		(53,654)	(39,881)	[1]
Accounts payable		(970)	1,789	[1]
Income taxes, net		(5,550)	591	[1]
Accrued commissions and bonuses		(2,239)	(3,131)	[1]
Accrued payroll and vacation		6,931	5,922	[1]
Deferred revenue		8,714	9,190	[1]
Accrued expenses and other current liabilities		6,176	4,242	[1]
Net cash provided by operating activities		176,419	145,849	[1]
Cash flows from investing activities
Purchase of short-term investments from funds held for clients		(61,268)	(137,614)	[1]
Proceeds from maturities of short-term investments from funds held for clients		54,200	95,500	[1]
Purchases of property and equipment		(71,080)	(44,264)	[1]
Net cash used in investing activities		(78,148)	(86,378)	[1]
Cash flows from financing activities
Repurchases of common stock	[1]		(42,857)
Withholding taxes paid related to net share settlement		(42,229)	(20,115)	[1]
Payments on long-term debt		(1,331)	(444)	[1]
Net change in client funds obligation		(131,869)	(186,454)	[1]
Payment of debt issuance costs		(16)	(58)	[1]
Net cash used in financing activities		(175,445)	(249,928)	[1]
Decrease in cash, cash equivalents, restricted cash and restricted cash equivalents		(77,174)	(190,457)	[1]
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period		986,464	1,098,860	[1]
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period		909,290	908,403	[1]
Cash and cash equivalents		108,127	85,048	[1]
Restricted cash included in funds held for clients		$ 801,163	$ 823,355	[1]

[1] Amounts have been adjusted to reflect the adoption of Accounting Standards Update No. 2016-18, “Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force).” See Note 2 for a summary of reclassifications and adjustments.