Fair Value Measurements - Fair Value and Significant Unobservable Inputs Used in Pricing Debt Securities (Details) - Recurring - Significant Unobservable Inputs (Level 3) $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 823
Fair Value		$ 523
Debt securities \| Securities Not Priced by the Company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	1
Fair Value		0
Corporate and government debt securities \| Securities Priced by the Company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 696
Fair Value		$ 385
Corporate and government debt securities \| Securities Priced by the Company \| Minimum \| Liquidity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0060
Unobservable Adjustment		0.0070
Corporate and government debt securities \| Securities Priced by the Company \| Maximum \| Liquidity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.1370
Unobservable Adjustment		0.0930
Corporate and government debt securities \| Securities Priced by the Company \| Weighted Average \| Liquidity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0470
Unobservable Adjustment		0.0280
Mortgage and other asset-backed securities \| Securities Priced by the Company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 126
Fair Value		$ 138
Mortgage and other asset-backed securities \| Securities Priced by the Company \| Minimum \| Liquidity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0060
Unobservable Adjustment		0.0060
Mortgage and other asset-backed securities \| Securities Priced by the Company \| Minimum \| Weighting of credit spreads
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0300
Unobservable Adjustment		0.0240
Mortgage and other asset-backed securities \| Securities Priced by the Company \| Maximum \| Liquidity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0380
Unobservable Adjustment		0.0370
Mortgage and other asset-backed securities \| Securities Priced by the Company \| Maximum \| Weighting of credit spreads
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0670
Unobservable Adjustment		0.0460
Mortgage and other asset-backed securities \| Securities Priced by the Company \| Weighted Average \| Liquidity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0080
Unobservable Adjustment		0.0070
Mortgage and other asset-backed securities \| Securities Priced by the Company \| Weighted Average \| Weighting of credit spreads
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0480
Unobservable Adjustment		0.0330

Debt Securities, Available-For-Sale, Including Disposal Group, Measurement Input

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- Definition

Marketable Securities Before Reclassification To Disposal Group Assets Held For Sale

+ References

No definition available.

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- Definition

Value of input used to measure investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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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.

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- Definition

Amount of investment in marketable security.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

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