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Mortgage-Backed Securities - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]      
Sale of securities $ 1,170,362 $ 903,822  
Unrealized gain in other comprehensive income     $ 30,000
Unrealized loss,on securities recognized through earnings 60,000    
Agency MBS      
Schedule Of Available For Sale Securities [Line Items]      
Sale of securities 1,000,000 903,800  
Gross realized losses on sales of securities   8,800  
Gross realized gains on sales of securities 9,700 2,600  
Unrealized gain (loss) on trading investments (1,141) 14,906  
Non-Agency MBS      
Schedule Of Available For Sale Securities [Line Items]      
Sale of securities 200,000    
Securities called   20,000  
Gross realized losses on sales of securities 55,400    
Gross realized gains on sales of securities   $ 22  
Unrealized gain (loss) on trading investments (59,982)    
Trading Securities. | Agency MBS      
Schedule Of Available For Sale Securities [Line Items]      
Balance on unrealized loss on Trading Agency MBS $ 0