Financial Guarantee Insurance Contracts - Estimated Future Amortization Expense for Insurance Intangible Asset (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Amortization Of Intangible Assets [Line Items]
Amortization of intangible assets	$ 14	$ 14	$ 33	$ 27
2019	20		20
2020	37		37
2021	34		34
2022	31		31
2023	28		28
Thereafter	228		228
Intangible assets	377		$ 377		$ 409
Insurance Intangible Asset [Member]
Amortization Of Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			7 years 4 months 24 days
Amortization of intangible assets	13	$ 14	$ 31	$ 27
Intangible Assets, Gross (Excluding Goodwill)	1,283		1,283		1,281
Finite-Lived Intangible Assets, Accumulated Amortization	940		940		908
2019	19		19
2020	34		34
2021	31		31
2022	28		28
2023	26		26
Thereafter	205		205
Intangible assets	$ 343		$ 343		$ 373

Amortization Of Intangible Assets [Line Items]

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Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

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Amount of amortization for asset, excluding financial asset and goodwill, lacking physical substance with finite life expected to be recognized after fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in remainder of current fiscal year.

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Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount before accumulated amortization of intangible assets, excluding goodwill.

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Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

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