Derivative Instruments (Open Credit Default Swap Liabilities) (Details) - Credit Default Swap, Seller [Member] $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security	Mar. 31, 2019 USD ($)
Summary Of Credit Derivatives
Maximum potential payout	$ 394		$ 55
BBB+ [Member]
Summary Of Credit Derivatives
Maturity	Jun. 20, 2025	Dec. 20, 2024
Credit rating of underlying obligation	BBB+	BBB+
Number of instruments \| security	4	1
Fair value	$ (2)	$ 1
Maximum potential payout	$ 394	$ 55

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- Definition

Maximum potential amount of future undiscounted payments that could be required under the credit derivative, before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4K
-Subparagraph (b)(1)
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- Definition

Fair value of credit risk derivative asset after deduction of credit risk derivative liability.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
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-Paragraph 4K
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- Definition

Description of the credit rating indicative of likelihood of repayment of the debt instrument given by a credit rating agency (for example, Standard and Poor's, Moody's, Fitch).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
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-Topic 505
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- Definition

Date the derivative contract matures, in CCYY-MM-DD format.

+ References

No definition available.

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Number of credit risk derivative instruments held.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
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