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Investments - Additional Information (Detail) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Schedule Of Investments [Line Items]                      
Equity method investments $ 37,659,000       $ 40,000,000       $ 37,659,000 $ 40,000,000  
Gains (losses) on equity method investments 1,354,000 $ 1,527,000 $ 1,119,000 $ 1,023,000 1,064,000 $ 1,530,000 $ 738,000 $ 783,000 5,023,000 4,115,000 $ 7,377,000
Impairment charge on equity method investments                 3,900,000 0 $ 0
Investments 38,008,000       40,349,000       38,008,000 40,349,000  
Consolidated VIE, total assets 3,949,300,000       3,926,914,000       3,949,300,000 3,926,914,000  
Consolidated VIE, total liabilities 3,120,397,000       3,178,308,000       3,120,397,000 3,178,308,000  
Maximum exposure to loss 4,800,000       2,800,000       4,800,000 2,800,000  
Variable Interest Entity, Primary Beneficiary [Member]                      
Schedule Of Investments [Line Items]                      
Consolidated VIE, total assets 7,200,000       7,400,000       7,200,000 7,400,000  
Consolidated VIE, total liabilities 1,000,000.0       2,100,000       1,000,000.0 2,100,000  
Equity Investments [Member]                      
Schedule Of Investments [Line Items]                      
Investments $ 400,000       $ 400,000       400,000 400,000  
Gains (losses) on equity method investments                 0 0  
Impairment charges, alternative measurements                 0 0  
Unrealized gains (losses) to reflect observable transactions                 $ (400,000) $ 18,200,000