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Fair value measurements - Narrative (Detail)
1 Months Ended 3 Months Ended 9 Months Ended
Jan. 31, 2021
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Nov. 04, 2021
company
Dec. 31, 2020
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Realized gain (loss) on available-for-sale securities   $ 0 $ 0 $ 0 $ 0    
Interest receivable   1,200,000   1,200,000     $ 3,100,000
Interest receivable, write-offs   0 0 0 0    
Investment at fair value   3,100,000   3,100,000     $ 5,653,000
Proceeds from sale of equity securities $ 31,300,000            
Subsequent Event              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Number of independent publicly traded companies | company           2  
Other (Expense) Income, Net              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Unrealized gain (loss) on equity securities   $ (500,000) $ 5,800,000 $ (28,800,000) $ 9,100,000