Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Cash flows from operating activities
Net income	$ 502.0	$ 340.8	$ 281.4
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	145.9	113.9	92.7
Accretion of discount on available-for-sale securities	(0.1)	(0.5)	(1.0)
Non-cash marketing expense	1.6	1.7	1.7
Gain on disposition of property and equipment			(0.2)
Amortization of debt issuance costs	1.1	1.2	0.8
Stock-based compensation expense	(22.9)	129.8	94.9
Loss on extinguishment of debt		1.2
Cash paid for derivative settlement			2.0
Gain on derivative			(1.6)
Deferred income taxes, net	5.8	2.6	(3.2)
Other	(0.5)	0.1	(2.0)
Changes in operating assets and liabilities:
Accounts receivable	(22.8)	6.4	(13.4)
Prepaid expenses	(6.7)	(6.6)	(6.5)
Inventory	0.0	0.2	(0.2)
Other assets	(2.7)	(9.6)	(4.0)
Deferred contract costs	(120.0)	(127.7)	(122.4)
Accounts payable	9.1	(5.2)	11.7
Income taxes, net	6.5	(12.8)	10.8
Accrued commissions and bonuses	2.5	2.1	6.1
Accrued payroll and vacation	2.9	11.1	10.8
Deferred revenue	14.1	13.1	16.0
Net change in operating right-of-use assets and operating lease liabilities	0.7	1.0	0.2
Accrued expenses and other current liabilities	17.4	22.2	(9.5)
Net cash provided by operating activities	533.9	485.0	365.1
Cash flows from investing activities
Purchases of investments from funds held for clients	(24.9)	(25.0)	(268.7)
Proceeds from investments from funds held for clients	200.0	25.0	382.2
Purchases of intangible assets	(4.4)	(4.2)	(4.1)
Purchases of property and equipment	(192.9)	(192.6)	(132.7)
Proceeds from sale of property and equipment		0.1
Net cash (used in) investing activities	(22.2)	(196.7)	(23.3)
Cash flows from financing activities
Proceeds from the issuance of debt			29.0
Repurchases of common stock	(122.8)	(286.6)	(94.7)
Withholding taxes paid related to net share settlements	(21.7)	(13.9)	(5.2)
Payments on long-term debt		(29.0)	(29.3)
Dividends paid	(84.8)	(64.8)
Net change in client funds obligation	1,337.6	120.4	361.1
Payment of debt issuance costs		(0.7)	(6.3)
Net cash (used in) provided by financing activities	1,108.3	(274.6)	254.6
Increase in cash, cash equivalents, restricted cash and restricted cash equivalents	1,620.0	13.7	596.4
Cash, cash equivalents, restricted cash and restricted cash equivalents
Cash, cash equivalents, restricted cash and restricted cash equivalents, beginning of period	2,422.8	2,409.1	1,812.7
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period	4,042.8	2,422.8	2,409.1
Cash and cash equivalents	402.0	294.0	400.7
Restricted cash included in funds held for clients	3,640.8	2,128.8	2,008.4
Total cash, cash equivalents, restricted cash and restricted cash equivalents, end of period	4,042.8	2,422.8	2,409.1
Supplemental disclosures of cash flow information:
Cash paid for interest, net of amounts capitalized	2.0	1.0	0.5
Cash paid for income taxes	136.9	139.9	100.6
Non-cash investing and financing activities:
Purchases of property and equipment, accrued but not paid	3.9	9.0	5.9
Stock-based compensation for capitalized software	17.5	14.7	9.0
Right of use assets obtained in exchange for operating lease liabilities	$ 25.1	$ 50.3	$ 21.5