Investments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions		Dec. 31, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Deferred compensation liabilities	[1]	$ (11.7)	$ (13.6)
Total		2,250.1	715.8
Quoted prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Investments [Line Items]
Deferred compensation liabilities	[1]	(11.7)	(13.6)
Total		2,250.1	715.8
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 Unobservable Inputs (Level 3) [Member]
Schedule of Investments [Line Items]
Deferred compensation liabilities	[1]	0.0	0.0
Total		0.0	0.0
Money Market Funds [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[2]	2,212.6	674.5
Money Market Funds [Member] | Quoted prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[2]	2,212.6	674.5
Money Market Funds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[2]	0.0	0.0
Money Market Funds [Member] | Significant Unobservable Inputs (Level 3) [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[2]	0.0	0.0
Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	23.1	24.3
Marketable Equity Securities [Member] | Quoted prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	23.1	24.3
Marketable Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	0.0	0.0
Marketable Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	0.0	0.0
Seed Capital Investments [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	26.1	30.6
Seed Capital Investments [Member] | Quoted prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	26.1	30.6
Seed Capital Investments [Member] | Significant Other Observable Inputs (Level 2) [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	[3]	0.0	0.0
Seed Capital Investments [Member] | Significant Unobservable Inputs (Level 3) [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 Consolidated Balance Sheet.

[2] 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 Consolidated Balance Sheet. As of December 31, 2022, included $156.5 million of cash and cash equivalents, $2.2 million of restricted cash and $515.8 million of funds receivable and funds held on behalf of clients on the Consolidated Balance Sheet.

[3] Included in investments on the Consolidated Balance Sheet.