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Investments (Narrative) (Detail)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2020
USD ($)
security
Dec. 31, 2019
USD ($)
security
Schedule Of Investments [Line Items]    
Fair value of securities on deposit with various regulatory authorities $ 11 $ 11
Fair value of securities pledged as collateral $ 299 $ 313
Number of securities in unrealized loss position for a continuous 12 month period | security 42 63
Rate that a security's fair value is below book value 5.00% 5.00%
Collateral Pledged [Member]    
Schedule Of Investments [Line Items]    
Cash $ 8  
Securities In Unrealized Loss Position [Member]    
Schedule Of Investments [Line Items]    
Weighted average contractual maturity period in years for securities in an unrealized loss position 8 years 10 years
Fair Value Below Book Value Greater Than Five Percent [Member]    
Schedule Of Investments [Line Items]    
Number of securities in unrealized loss position for a continuous 12 month period | security 41 16