Investments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Investments measured at fair value		$ 21.2
Deferred compensation liabilities	$ (21.2)	(20.3)
Total	1,907.2	737.8
Money Market Funds [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	1,862.4	698.9
Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	40.9	38.0
Seed Capital Investments [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	25.1
Quoted prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Investments [Line Items]
Investments measured at fair value		21.2
Deferred compensation liabilities	(21.2)	(20.3)
Total	1,907.2	737.8
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	1,862.4	698.9
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	40.9	$ 38.0
Quoted prices in Active Markets for Identical Assets (Level 1) [Member] | Seed Capital Investments [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	$ 25.1