Consolidated Statements of Cash Flows - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Cash flows from operating activities:
Net income		$ 180,576	$ 137,065	[1]	$ 123,486	[1]
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		42,211	29,657	[1]	19,395	[1]
Accretion of discount on available-for-sale securities		(940)	(1,112)	[1]	(451)	[1]
Loss on disposition of property and equipment	[1]				21
Amortization of debt discount and debt issuance costs		35	32	[1]	117	[1]
Stock-based compensation expense		47,268	36,576	[1]	36,076	[1]
Loss on early repayment of debt	[1]				923
Cash paid for derivative settlement		(81)	(188)	[1]	(24)	[1]
(Gain)/loss on derivative		1,456	(479)	[1]	673	[1]
Deferred income taxes, net		21,011	21,077	[1]	(7,681)	[1]
Changes in operating assets and liabilities:
Accounts receivable		(5,884)	(1,838)	[1]	(237)	[1]
Prepaid expenses		(5,903)	(2,676)	[1]	(507)	[1]
Inventory		(403)	(306)	[1]	462	[1]
Other assets		(3,555)	(762)	[1]	(241)	[1]
Deferred contract costs		(76,204)	(60,730)	[1]	(48,619)	[1]
Accounts payable		(221)	1,079	[1]	79	[1]
Income taxes, net		(58)	3,085	[1]	(6,355)	[1]
Accrued commissions and bonuses		1,672	1,086	[1]	1,582	[1]
Accrued payroll and vacation		4,129	3,726	[1]	2,246	[1]
Deferred revenue		11,593	13,027	[1]	11,913	[1]
Accrued expenses and other current liabilities		7,561	6,498	[1]	(2,709)	[1]
Net cash provided by operating activities		224,263	184,817	[1]	130,149	[1]
Cash flows from investing activities:
Purchase of short-term investments from funds held for clients		(195,811)	(145,011)	[1]	(66,235)	[1]
Proceeds from maturities of short-term investments from funds held for clients		69,200	155,500	[1]	141,205	[1]
Purchases of property and equipment		(92,934)	(59,906)	[1]	(59,389)	[1]
Net cash provided by (used in) investing activities		(219,545)	(49,417)	[1]	15,581	[1]
Cash flows from financing activities:
Proceeds from issuance of long-term debt	[1]				40,940
Repurchases of common stock	[1]		(105,188)		(56,880)
Withholding taxes paid related to net share settlements		(42,528)	(20,248)	[1]	(32,850)	[1]
Payments on long-term debt		(1,775)	(888)	[1]	(35,335)	[1]
Net change in client funds obligation		694,991	(121,414)	[1]	230,957	[1]
Debt extinguishment costs	[1]				(823)
Payment of debt issuance costs		(16)	(58)	[1]	(344)	[1]
Net cash provided by (used in) financing activities		650,672	(247,796)	[1]	145,665	[1]
Increase (decrease) in cash, cash equivalents, restricted cash and restricted cash equivalents		655,390	(112,396)	[1]	291,395	[1]
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period	[1]	986,464	1,098,860		807,465
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period		1,641,854	986,464	[1]	1,098,860	[1]
Cash and cash equivalents		133,667	45,718	[1]	46,077	[1]
Restricted cash included in funds held for clients		1,508,187	940,746	[1]	1,052,783	[1]
Supplemental disclosures of cash flow information:
Cash paid for interest, net of amounts capitalized		891	708	[1]	791	[1]
Cash paid for income taxes		24,566	13,511	[1]	18,332	[1]
Noncash investing and financing activities:
Purchases of property and equipment, accrued but not paid		7,451	1,759	[1]	6,686	[1]
Stock-based compensation for capitalized software		$ 4,757	$ 3,722	[1]	$ 3,285	[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.