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Derivatives and Hedging Activities - Schedule of Credit Derivatives (Detail) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Credit Derivatives    
Notional amount $ 14,488,077 $ 14,609,961
GSE CRT | Embedded Credit Derivative    
Credit Derivatives    
Fair value amount 10,281 22,771
Notional amount 464,966 526,912
Maximum potential amount of future undiscounted payments $ 464,966 $ 526,912