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Investments - Additional Information (Detail) - USD ($)
Jun. 30, 2020
Dec. 31, 2019
Schedule of Investments [Line Items]    
Debt Securities, Available-for-sale, Allowance for Credit Loss $ 0  
Other investments 401,000,000 $ 478,000,000
Securities fair value 7,000,000 6,000,000
Ambac Assurance [Member]    
Schedule of Investments [Line Items]    
Unsettled Amounts Secured Notes from LSNI 15,000,000 55,000,000
Interest Rate Contract [Member]    
Schedule of Investments [Line Items]    
Other investments 49,000,000 176,000,000
Reported Value Measurement [Member]    
Schedule of Investments [Line Items]    
Other investments 50,000,000 136,000,000
Reported Value Measurement [Member] | Interest Rate Contract [Member]    
Schedule of Investments [Line Items]    
Other investments 3,000,000 $ 81,000,000
Commitments [Member] | Partnership Interest [Member]    
Schedule of Investments [Line Items]    
Other investments $ 49,000,000