Fair value measurements - Narrative (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized gain (loss) recognized on sale		$ 0	$ 0
Interest receivable		3,100,000	3,600,000
Interest receivable, write-off		0	0
Investment at fair value		5,653,000	12,870,000
Subsequent Event
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment at fair value	$ 7,300,000
Proceeds from sale of equity securities	$ 31,300,000
Other Income (Expense), Net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) on equity securities		$ 7,200,000	$ 9,300,000	$ (2,200,000)