Contracts Accounted for as Insurance - Net Expected Loss to be Expensed Insurance Contracts (Details) $ in Millions	Dec. 31, 2018 USD ($)
Insurance [Abstract]
2019 (January 1 – March 31)	$ 8
2019 (April 1 – June 30)	10
2019 (July 1 – September 30)	9
2019 (October 1 – December 31)	9
Subtotal 2019	36
2020	37
2021	39
2022	40
2023	38
2024-2028	156
2029-2033	104
2034-2038	47
After 2038	10
Net expected loss to be expensed	507
Future accretion	88
Total expected future loss and LAE	$ 595

Financial Guarantee Insurance Contracts Future Expected Net Loss to be Expensed Beyond Twentieth Fiscal Year

No definition available.

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedBeyondTwentiethFiscalYear
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For financial guaranty insurance contracts, the amount of net loss expected to be expensed during the first quarter of the subsequent annual period following the statement of financial position reporting date.

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedDuringFirstQuarterFollowingBalanceSheetDate
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financial Guarantee Insurance Contracts Future Expected Net Loss to be Expensed During Fiscal Year Following Balance Sheet Date

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedDuringFiscalYearFollowingBalanceSheetDate
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financial Guarantee Insurance Contracts Future Expected Net Loss to be Expensed During Fourth Quarter Following Balance Sheet Date

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedDuringFourthQuarterFollowingBalanceSheetDate
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For financial guaranty insurance contracts, the amount of net loss expected to be expensed during the second quarter of the subsequent annual period following the statement of financial position reporting date.

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedDuringSecondQuarterFollowingBalanceSheetDate
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financial Guarantee Insurance Contracts Future Expected Net Loss to be Expensed During Third Quarter Following Balance Sheet Date

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedDuringThirdQuarterFollowingBalanceSheetDate
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financial Guarantee Insurance Contracts Future Expected Net Loss to be Expensed More than 10 and within 15 Years from Balance Sheet Date

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedMorethan10andwithin15YearsfromBalanceSheetDate
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financial Guarantee Insurance Contracts Future Expected Net Loss to be Expensed More than 15 and within 20 Years from Balance Sheet Date

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedMorethan15andwithin20YearsfromBalanceSheetDate
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For financial guarantee insurance contracts, the amount of net loss expected to be expensed during the sixth through tenth year following the statement of financial position reporting date.

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedMorethanFiveandWithin10YearsfromBalanceSheetDate
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financial Guarantee Insurance Contracts Future Expected Net Loss to be Expensed in Fifth Full Fiscal Year

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedinFifthFullFiscalYear
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For financial guarantee insurance contracts, the amount of net loss expected to be expensed within the fourth full fiscal year following the date of the latest balance sheet presented in the financial statements.

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedinFourthFullFiscalYear
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For financial guaranty insurance contracts, the amount of net loss expected to be expensed within the second full fiscal year following the date of the latest balance sheet presented in the financial statements.

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedinSecondFullFiscalYear
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financial Guarantee Insurance Contracts, Future Expected, Net Loss to be Expensed in Third Full Fiscal Year

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetLosstobeExpensedinThirdFullFiscalYear
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financial Guarantee Insurance Contracts, Net Loss to be Expensed, Present Value

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsNetLosstobeExpensedPresentValue
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financial Guarantee Insurance Contracts, Net Loss to be Expensed, Undiscounted

+ References

No definition available.

+ Details

Name:	ago_FinancialGuaranteeInsuranceContractsNetLosstobeExpensedUndiscounted
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Schedule of Insured Financial Obligations with Credit Deterioration, Reductions of Gross Claim Liability, Net of Reinsurance Recoverable, Discount

+ References

No definition available.

+ Details

Name:	ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetofReinsuranceRecoverableDiscount
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_InsuranceAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration