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Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Details) - USD ($) | Sep. 30, 2020 | Dec. 31, 2019
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Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Unrealized Loss Position | $ 515,000,000 | $ 489,000,000
Debt Securities, Available-for-sale, Amortized Cost | 2,914,000,000 | 3,187,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 141,000,000 | 132,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 490,000,000 | 401,000,000
Non - credit other - than - temporary Impairments |  | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 5,000,000 | 4,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 25,000,000 | 88,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 1,000,000 | 1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 6,000,000 | 5,000,000
Available-for-sale Securities, Current | 3,048,000,000 | 3,314,000,000
Fixed income securities, at fair value | 3,048,000,000 |
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Unrealized Loss Position | 152,000,000 | 201,000,000
Debt Securities, Available-for-sale, Amortized Cost | 586,000,000 | 653,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 152,000,000 | 201,000,000
Non - credit other - than - temporary Impairments |  | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Fixed income securities, at fair value | 586,000,000 | 653,000,000
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Unrealized Loss Position | 363,000,000 | 288,000,000
Debt Securities, Available-for-sale, Amortized Cost | 2,176,000,000 | 2,450,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 140,000,000 | 132,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 338,000,000 | 200,000,000
Non - credit other - than - temporary Impairments |  | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 5,000,000 | 4,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 25,000,000 | 88,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 1,000,000 | 1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 6,000,000 | 5,000,000
Fixed income securities, at fair value | 2,311,000,000 | 2,577,000,000
Fixed Income Securities [Member] | Municipal Bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Unrealized Loss Position | 70,000,000 | 23,000,000
Debt Securities, Available-for-sale, Amortized Cost | 322,000,000 | 194,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 29,000,000 | 22,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 64,000,000 | 13,000,000
Non - credit other - than - temporary Impairments |  | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1,000,000 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 6,000,000 | 10,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1,000,000 | 0
Fixed income securities, at fair value | 351,000,000 | 215,000,000
Fixed Income Securities [Member] | Corporate Debt Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Unrealized Loss Position | 179,000,000 | 68,000,000
Debt Securities, Available-for-sale, Amortized Cost | 1,066,000,000 | 1,396,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 28,000,000 | 36,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 179,000,000 | 63,000,000
Non - credit other - than - temporary Impairments |  | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 3,000,000 | 2,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 5,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 3,000,000 | 2,000,000
Fixed income securities, at fair value | 1,091,000,000 | 1,430,000,000
Fixed Income Securities [Member] | Foreign Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Unrealized Loss Position | 12,000,000 | 20,000,000
Debt Securities, Available-for-sale, Amortized Cost | 76,000,000 | 44,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 1,000,000 | 1,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 12,000,000 | 20,000,000
Non - credit other - than - temporary Impairments |  | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Fixed income securities, at fair value | 77,000,000 | 44,000,000
Fixed Income Securities [Member] | US Government Debt Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Unrealized Loss Position | 11,000,000 | 38,000,000
Debt Securities, Available-for-sale, Amortized Cost | 118,000,000 | 157,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 4,000,000 | 2,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 11,000,000 | 36,000,000
Non - credit other - than - temporary Impairments |  | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1,000,000 | 2,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 2,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1,000,000 | 2,000,000
Fixed income securities, at fair value | 122,000,000 | 156,000,000
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Unrealized Loss Position | 19,000,000 | 5,000,000
Debt Securities, Available-for-sale, Amortized Cost | 251,000,000 | 200,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 45,000,000 | 47,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 19,000,000 | 5,000,000
Non - credit other - than - temporary Impairments |  | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Fixed income securities, at fair value | 296,000,000 | 248,000,000
Fixed Income Securities [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 49,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss |  | 1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 50,000,000
Fixed income securities, at fair value |  | 0
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Unrealized Loss Position | 67,000,000 | 116,000,000
Debt Securities, Available-for-sale, Amortized Cost | 74,000,000 | 147,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 53,000,000 | 53,000,000
Non - credit other - than - temporary Impairments |  | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1,000,000 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 15,000,000 | 63,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 1,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1,000,000 | 1,000,000
Fixed income securities, at fair value | 73,000,000 | 146,000,000
Fixed Income Securities [Member] | Asset-backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Unrealized Loss Position | 5,000,000 | 10,000,000
Debt Securities, Available-for-sale, Amortized Cost | 268,000,000 | 263,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 33,000,000 | 24,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 1,000,000 | 2,000,000
Non - credit other - than - temporary Impairments |  | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 4,000,000 | 7,000,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 1,000,000 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1,000,000 | 0
Fixed income securities, at fair value | 300,000,000 | 287,000,000
Fixed Income Investments And Other Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 2,761,000,000 | 3,103,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 132,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 141,000,000 | 5,000,000
Non - credit other - than - temporary Impairments |  | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 6,000,000 |
Fixed income securities, at fair value | 2,896,000,000 | 3,230,000,000
Collateral Pledged [Member] | Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 0
Non - credit other - than - temporary Impairments |  | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 0
Fixed income securities, at fair value |  | 85,000,000
Collateral Pledged [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Fixed income securities, at fair value | 152,000,000 | 85,000,000
Collateral Pledged [Member] | Fixed Income Securities [Member] | Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 152,000,000 | 85,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 0 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 |
Fixed income securities, at fair value | 152,000,000 |
Asset Pledged as Collateral [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 152,000,000 | $ 85,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss | $ 0 |

X
- Definition Accumulated other comprehensive loss related to portion of other than temporary impairment (OTTI) due to factors other than credit losses on debt securities categorized as available-for-sale that an entity does not intend to sell and which it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis of the debt security. The amount represented by this item is net of tax, as applicable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 9A -URI http://asc.fasb.org/extlink&oid=118954083&loc=SL6284427-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aaa) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669686-108580 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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