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Fair Value Measurements - Reconciliation of Our Investment in Restricted Equity Securities Measured at Fair Value on Quarterly Basis Using Significant Unobservable Inputs (Level 3) (Detail) - Level 3 - Fair Value, Recurring - Equity securities–biotechnology industry - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2022
Jun. 30, 2021
Jun. 30, 2022
Jun. 30, 2021
Fair Value, Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Roll Forward]        
Balance at beginning of period $ 0.0 $ 38.9 $ 11.0 $ 38.2
Unrealized gain included in earnings 0.0 0.0 20.8 0.7
Transfers out of Level 3 0.0 0.0 (31.8) 0.0
Balance at end of period $ 0.0 $ 38.9 $ 0.0 $ 38.9