Investments - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Schedule Of Investments [Line Items]
Equity method investments	$ 39,800,000		$ 39,800,000		$ 40,000,000.0
Gains (losses) on equity method investments	1,527,000	$ 1,530,000	3,669,000	$ 3,051,000
Impairment charge on equity method investments	200,000	0	2,700,000	0
Investments	40,169,000		40,169,000		40,349,000
Consolidated VIE, total assets	4,594,285,000		4,594,285,000		3,916,120,000
Consolidated VIE, total liabilities	3,768,380,000		3,768,380,000		3,148,745,000
Maximum exposure to loss	3,700,000		3,700,000		2,800,000
Variable Interest Entity, Primary Beneficiary [Member]
Schedule Of Investments [Line Items]
Consolidated VIE, total assets	6,900,000		6,900,000		7,400,000
Consolidated VIE, total liabilities	1,700,000		1,700,000		2,100,000
Equity Investments [Member]
Schedule Of Investments [Line Items]
Investments	400,000		400,000		$ 400,000
Gains (losses) on equity method investments	0	0	0	0
Impairment charges, alternative measurements	0	0	0	0
Unrealized gains (losses) to reflect observable transactions	$ (1,000,000.0)	$ 0	$ (1,000,000.0)	$ 20,400,000