Investments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Deferred compensation liabilities	$ (14.1)	$ (20.8)
Total	673.7	1,068.2
Money Market Funds [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	658.0	1,051.1
Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Investments measured at fair value	29.8	37.9
Quoted prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Investments [Line Items]
Deferred compensation liabilities	(14.1)	(20.8)
Total	673.7	1,068.2
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	658.0	1,051.1
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	$ 29.8	$ 37.9