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Investments - Securities Available-for-Sale in Gross Unrealized Loss Position (Detail) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Corporate Bonds and Other [Member]    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value, Less than 12 Months $ 1,656 $ 8,543
Gross Unrealized Losses, Less than 12 Months 39 340
Estimated Fair Value, 12 Months or Longer 1,743 825
Gross Unrealized Losses, 12 Months or Longer 60 55
Total Estimated Fair Value 3,399 9,368
Total Gross Unrealized Losses 99 395
States, Municipalities and Political Subdivisions [Member]    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value, Less than 12 Months 14 517
Gross Unrealized Losses, Less than 12 Months   8
Estimated Fair Value, 12 Months or Longer 3 5
Gross Unrealized Losses, 12 Months or Longer   1
Total Estimated Fair Value 17 522
Total Gross Unrealized Losses   9
Residential Mortgage-Backed [Member]    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value, Less than 12 Months 94 1,932
Gross Unrealized Losses, Less than 12 Months   23
Estimated Fair Value, 12 Months or Longer 1,494 1,119
Gross Unrealized Losses, 12 Months or Longer 20 34
Total Estimated Fair Value 1,588 3,051
Total Gross Unrealized Losses 20 57
Commercial Mortgage-Backed [Member]    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value, Less than 12 Months 184 728
Gross Unrealized Losses, Less than 12 Months 2 10
Estimated Fair Value, 12 Months or Longer 236 397
Gross Unrealized Losses, 12 Months or Longer 5 22
Total Estimated Fair Value 420 1,125
Total Gross Unrealized Losses 7 32
Other Asset-Backed [Member]    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value, Less than 12 Months 450 834
Gross Unrealized Losses, Less than 12 Months 10 21
Estimated Fair Value, 12 Months or Longer 95 125
Gross Unrealized Losses, 12 Months or Longer 2 3
Total Estimated Fair Value 545 959
Total Gross Unrealized Losses 12 24
Total Asset-Backed [Member]    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value, Less than 12 Months 728 3,494
Gross Unrealized Losses, Less than 12 Months 12 54
Estimated Fair Value, 12 Months or Longer 1,825 1,641
Gross Unrealized Losses, 12 Months or Longer 27 59
Total Estimated Fair Value 2,553 5,135
Total Gross Unrealized Losses 39 113
U.S. Treasury and obligations of government-sponsored enterprises [Member]    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value, Less than 12 Months 48 21
Gross Unrealized Losses, Less than 12 Months 1  
Estimated Fair Value, 12 Months or Longer 14 19
Total Estimated Fair Value 62 40
Total Gross Unrealized Losses 1  
Foreign Government [Member]    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value, Less than 12 Months 32 114
Gross Unrealized Losses, Less than 12 Months 1 2
Estimated Fair Value, 12 Months or Longer 27 124
Gross Unrealized Losses, 12 Months or Longer   2
Total Estimated Fair Value 59 238
Total Gross Unrealized Losses 1 4
Fixed Maturity Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value, Less than 12 Months 2,478 12,689
Gross Unrealized Losses, Less than 12 Months 53 404
Estimated Fair Value, 12 Months or Longer 3,612 2,614
Gross Unrealized Losses, 12 Months or Longer 87 117
Total Estimated Fair Value 6,090 15,303
Total Gross Unrealized Losses $ 140 $ 521