Investments - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Schedule Of Investments [Line Items]
Equity method investments	$ 38,400,000		$ 38,400,000		$ 40,000,000.0
Gains (losses) on equity method investments	1,119,000	$ 738,000	2,142,000	$ 1,521,000
Impairment charge on equity method investments	0	0	2,500,000	0
Investments	38,751,000		38,751,000		40,349,000
Consolidated VIE, total assets	4,502,354,000		4,502,354,000		3,916,120,000
Consolidated VIE, total liabilities	3,716,333,000		3,716,333,000		3,148,745,000
Maximum exposure to loss	1,100,000		1,100,000		2,800,000
Variable Interest Entity, Primary Beneficiary [Member]
Schedule Of Investments [Line Items]
Consolidated VIE, total assets	6,700,000		6,700,000		7,400,000
Consolidated VIE, total liabilities	2,600,000		2,600,000		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	$ (5,900)	$ 20,400,000	$ (46,200)	$ 20,600,000