Investments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions		Mar. 31, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Deferred compensation liabilities	[1]	$ (12.3)	$ (11.7)
Total		1,818.1	2,250.1
Money Market Funds [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[2]	1,781.6	2,212.6
Seed Capital Investments
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	26.3	26.1
Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	22.5	23.1
Quoted prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Investments [Line Items]
Deferred compensation liabilities	[1]	(12.3)	(11.7)
Total		1,818.1	2,250.1
Quoted prices in Active Markets for Identical Assets (Level 1) [Member] | Money Market Funds [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[2]	1,781.6	2,212.6
Quoted prices in Active Markets for Identical Assets (Level 1) [Member] | Seed Capital Investments
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	26.3	26.1
Quoted prices in Active Markets for Identical Assets (Level 1) [Member] | Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	22.5	23.1
Significant Other Observable Inputs (Level 2) [Member]
Schedule of Investments [Line Items]
Deferred compensation liabilities	[1]	0.0	0.0
Total		0.0	0.0
Significant Other Observable Inputs (Level 2) [Member] | Money Market Funds [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[2]	0.0	0.0
Significant Other Observable Inputs (Level 2) [Member] | Seed Capital Investments
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	0.0	0.0
Significant Other Observable Inputs (Level 2) [Member] | Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	0.0	0.0
Significant Unobservable Inputs (Level 3) [Member]
Schedule of Investments [Line Items]
Deferred compensation liabilities	[1]	0.0	0.0
Total			0.0
Significant Unobservable Inputs (Level 3) [Member] | Money Market Funds [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[2]	0.0	0.0
Significant Unobservable Inputs (Level 3) [Member] | Seed Capital Investments
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	0.0	0.0
Significant Unobservable Inputs (Level 3) [Member] | Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	$ 0.0	$ 0.0

[1] Included in Other long-term liabilities on the Condensed Consolidated Balance Sheet

[2] As of March 31, 2024, included $112.3 million of cash and cash equivalents, $1.6 million of restricted cash and $1,667.7 million of funds receivable and funds held on behalf of clients on the Condensed Consolidated Balance Sheet. As of December 31, 2023, included $131.7 million of cash and cash equivalents, $1.8 million of restricted cash and $2,079.1 million of funds receivable and funds held on behalf of clients on the Condensed Consolidated Balance Sheet.

[3] Included in Investments on the Condensed Consolidated Balance Sheet