Quarterly report pursuant to Section 13 or 15(d)

Short-term and Long-term Investments, Restricted Deposits - Narrative (Details)

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Short-term and Long-term Investments, Restricted Deposits - Narrative (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2020
USD ($)
position
Dec. 31, 2019
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Accrued interest income | $ $ 78  
Positions from which gross unrealized losses were generated 2,636  
Total unrealized investment positions 5,998  
Commercial mortgage-backed securities    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Investments recorded at fair value that carry rating of AA Plus, weighted average (in years)   4 years
Rated Securities | External Credit Rating, Investment Grade    
Schedule Of Investments And Restricted Deposits By Type [Line Items]    
Percentage of investments in rated securities carry an investment grade rating by nationally recognized statistical rating organizations 97.00%