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
	Mar. 31, 2025	Mar. 31, 2024
Revenues
Total revenues	$ 530.5	$ 499.9
Cost of revenues
Operating expenses	66.3	63.7
Depreciation and amortization	18.3	15.0
Total cost of revenues	84.6	78.7
Gross profit	445.9	421.2
Administrative expenses
Sales and marketing	110.9	115.5
Research and development	62.3	50.5
General and administrative	66.0	(48.1)
Depreciation and amortization	21.6	17.5
Total administrative expenses	260.8	135.4
Total operating expenses	345.4	214.1
Operating income	185.1	285.8
Interest expense	(0.8)	(0.8)
Other income (expense), net	6.0	5.0
Income before income taxes	190.3	290.0
Provision for income taxes	50.9	42.8
Net income	139.4	247.2
Recurring [Member]
Revenues
Total revenues	493.4	460.0
Implementation and Other [Member]
Revenues
Total revenues	6.6	6.0
Interest on Funds Held for Clients [Member]
Revenues
Total revenues	$ 30.5	$ 33.9