Contracts Accounted for as Insurance - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Guarantor Obligations [Line Items]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	$ 674	$ 844
Weighted average risk-free rates for U.S. dollar denominated financial guaranty insurance obligations	2.74%	2.39%
Net par amount outstanding	$ 241,802	$ 264,952
Variable Rate Demand Obligation [Member]
Guarantor Obligations [Line Items]
Rate basis for bank bond rate	prime rate
Bonds held by bank, minimum installment payment period	5 years
Minimum [Member]
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Claim Liability, Risk Free Discount Rate	0.00%	0.00%
Minimum [Member] \| Variable Rate Demand Obligation [Member]
Guarantor Obligations [Line Items]
Bank bond rate (as a percent)	2.00%
Threshold period of bonds held by bank for right of accelerated repayment (in days)	90 days
Maximum [Member]
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Claim Liability, Risk Free Discount Rate	3.06%	2.78%
Maximum [Member] \| Variable Rate Demand Obligation [Member]
Guarantor Obligations [Line Items]
Bank bond rate (as a percent)	3.00%
Bank bond capped rate (as a percent)	25.00%
Threshold period of bonds held by bank for right of accelerated repayment (in days)	180 days
Termination of Swap Obligation due to Further Rating Downgrade [Member] \| Maximum [Member] \| AGM [Member]
Guarantor Obligations [Line Items]
Increase in losses as a result of an adverse outcome, minimum	$ 212
Variable Rate Demand Obligation [Member] \| AGM and AGC [Member]
Guarantor Obligations [Line Items]
Net par amount outstanding	4,500
Internal Credit, B Minus Rating [Member] \| Variable Rate Demand Obligation [Member] \| AGM and AGC [Member]
Guarantor Obligations [Line Items]
Net par amount outstanding	$ 53
Foreign Currency Concentration Risk [Member] \| Premiums Receivable [Member]
Guarantor Obligations [Line Items]
Percentage of installment premiums denominated in currencies other than the U.S. dollar	72.00%	72.00%
Variable Interest Entity, Primary Beneficiary [Member]
Guarantor Obligations [Line Items]
Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance and Salvage and Subrogation Recoveries	$ 47	$ 55

Financial Guarantee Insurance Contracts, Claim Liability, Risk Free Discount Rate

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- Definition

Represents the typical percentage points added to the reference rate on bonds insured by the entity and held by the bank.

+ References

No definition available.

+ Details

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- Definition

Represents the typical maximum interest rate on bonds insured by the entity and held by the bank.

+ References

No definition available.

+ Details

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- Definition

The reference rate for the variable interest rate typically accrued on bonds insured by the entity and held by the bank.

+ References

No definition available.

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- Definition

Financial Guaranty Insurance Contracts Threshold Of Bonds Held By Bank, Minimum Installment Payment Period

+ References

No definition available.

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- Definition

Represents the low end of the range of the period of time after which the bank has the right to demand accelerated repayment on bonds insured by the entity and held by the bank.

+ References

No definition available.

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- Definition

Represents the outstanding principal amounts of the underlying insured obligations for financial guaranty contracts, after netting out ceded contracts.

+ References

No definition available.

+ Details

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- Definition

Represents the reserve for losses and loss adjustment expenses, net of reinsurance recoverable on unpaid losses and net of salvage and subrogation recoverable.

+ References

No definition available.

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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- Definition

This element represents the weighted average risk free rate used to discount the claim liability related to financial guarantee insurance contracts.

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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

Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

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