Segment Reporting - Schedule of Company 's Revenues Expenses and Net (Loss) for Each Reportable Segment Consistant with Net Income (Loss) As Reported On Consolidated Statements of Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2025	Mar. 31, 2025	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2025	Jun. 30, 2024
Revenues
Total revenues	$ 483.6		$ 437.5		$ 1,014.1	$ 937.4
Cost of revenues
Operating expenses	68.4		67.5		134.8	131.1
Depreciation and amortization	19.1		16.4		37.4	31.4
Total cost of revenues	87.5		83.9		172.2	162.5
Gross profit	396.1		353.6		841.9	774.9
Administrative expenses
Sales and marketing	116.0		106.9		226.9	222.4
Research and development	74.8		62.4		137.1	112.9
General and administrative	70.2		70.1		135.9	22.0
Depreciation and amortization	22.8		19.2		44.5	36.7
Total administrative expenses	283.8		258.5		544.4	393.9
Total operating expenses	371.3		342.4		716.6	556.4
Operating income	112.3		95.1		297.5	381.0
Interest expense	(0.8)		(0.8)		(1.6)	(1.6)
Other income (expense), net	5.7		4.8		11.6	9.8
Income before income taxes	117.2		99.1		307.5	389.2
Provision for income taxes	27.7		31.2		78.6	74.0
Net income	89.5	$ 139.4	68.0	$ 247.2	228.9	315.2
Recurring [Member]
Revenues
Total revenues	449.1		399.6		942.5	859.6
Implementation and Other [Member]
Revenues
Total revenues	6.0		5.9		12.6	12.0
Interest on Funds Held for Clients [Member]
Revenues
Total revenues	$ 28.5		$ 32.0		$ 59.0	$ 65.8