Available-for-Sale Debt Securities and Short-Term Investments

No definition available.

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

- Definition

Represents the number of fixed maturity securities and short-term investments which have been valued using model processes set by the entity.

+ References

No definition available.

+ Details

Name:	ago_AvailableforsaleDebtSecuritiesNumberofInstruments
Namespace Prefix:	ago_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the gross spread allocated to bank profit. Bank profit represents the profit that the originator realizes for putting the deal together and funding the transaction.

+ References

No definition available.

+ Details

Name:	ago_CreditDerivativeBankProfit
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the bank profit expressed as a percentage of original or gross spread.

+ References

No definition available.

+ Details

Name:	ago_CreditDerivativeBankProfitAsPercentageOfOriginalGrossSpreadOrCashBondPrice
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the gross spread allocated to hedge cost. Hedge cost represents the cost of protection purchased on the entity by the originator to hedge their counterparty credit risk exposure to the entity.

+ References

No definition available.

+ Details

Name:	ago_CreditDerivativeHedgeCost
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the hedge cost expressed as a percentage of original or gross spread.

+ References

No definition available.

+ Details

Name:	ago_CreditDerivativeHedgeCostAsPercentageOfOriginalGrossSpreadOrCashBondPrice
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of sources on which credit spread is based.

+ References

No definition available.

+ Details

Name:	ago_CreditDerivativeNumberOfSourcesOfCreditSpread
Namespace Prefix:	ago_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the original gross spread/cash bond price. Gross spread is the difference between the yield of a security paid by an issuer on an insured versus uninsured basis or in the case of a CDS transaction the difference between the yield and an index such as LIBOR.

+ References

No definition available.

+ Details

Name:	ago_CreditDerivativeOriginalGrossSpreadOrCashBondPrice
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage of gross spread.

+ References

No definition available.

+ Details

Name:	ago_CreditDerivativeOriginalGrossSpreadPercentage
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage of exposure hedged by the bank or deal originator.

+ References

No definition available.

+ Details

Name:	ago_CreditDerivativePercentageOfExposureHedgedByOriginator
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the gross spread allocated to net spread. It represents the premiums received by the company for the credit protection provided. Such net spread varies based on the variation in the gross spread, bank profit and hedged cost.

+ References

No definition available.

+ Details

Name:	ago_CreditDerivativePremiumReceivedPerAnnum
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the premiums received per annum by the company expressed as a percentage of original or gross spread.

+ References

No definition available.

+ Details

Name:	ago_CreditDerivativePremiumReceivedPerAnnumAsPercentageOfOriginalGrossSpreadOrCashBondPrice
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

This element represents the fair value investments categorized as other which are not in and of themselves material enough to require separate disclosure.

+ References

No definition available.

+ Details

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

- Definition

Represents the percentage of CDS contracts which are fair valued using minimum premium. Minimum premium is the minimum rate that the company would currently charge to assume similar risks.

+ References

No definition available.

+ Details

Name:	ago_PercentageOfCdsContractsFairValuedUsingMinimumPremium
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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.

+ Details

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

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

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Period Type:	duration

- Details

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- Details

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- Details

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- Details

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Fair Value Measurement - Measured and Carried at Fair Value (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($) source Security	Dec. 31, 2016 USD ($)	Mar. 31, 2017
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Percentage of CDS contracts which are fair valued using minimum premium	34.00%	26.00%	39.00%
CDS contracts [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Gross spread percentage	100.00%	100.00%
Number of sources of credit spread \| source	3
Based on actual collateral specific spreads [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Gross spread percentage	8.00%	7.00%
Based on market indices [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Gross spread percentage	66.00%	77.00%
Provided by the CDS counterparty [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Gross spread percentage	26.00%	16.00%
Scenario 1 [Member] \| CDS contracts [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Bank profit as % of total	62.00%
Hedge cost as % of total	16.00%
Premium received per annum as % of total	22.00%
Original gross spread/cash bond price (as a percent)	1.85%
Bank profit (as a percent)	1.15%
Hedge cost (as a percent)	0.30%
The Company premium received per annum (as a percent)	0.40%
Scenario 1 [Member] \| CDS contracts [Member] \| AGC [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Original gross spread/cash bond price (as a percent)	3.00%
Percentage of exposure hedged	10.00%
Scenario 2 [Member] \| CDS contracts [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Bank profit as % of total	10.00%
Hedge cost as % of total	88.00%
Premium received per annum as % of total	2.00%
Original gross spread/cash bond price (as a percent)	5.00%
Bank profit (as a percent)	0.50%
Hedge cost (as a percent)	4.40%
The Company premium received per annum (as a percent)	0.10%
Scenario 2 [Member] \| CDS contracts [Member] \| AGC [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Original gross spread/cash bond price (as a percent)	17.60%
Percentage of exposure hedged	25.00%
Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets	$ 53	$ 52
Recurring [Member] \| Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Number of fixed maturity securities valued using model processes \| Security	79
Fixed maturity securities	$ 1,167
Recurring [Member] \| Level 3 [Member] \| CDS contracts [Member] \| Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount factor (as a percent)	1.16%	1.00%
Recurring [Member] \| Level 3 [Member] \| CDS contracts [Member] \| Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount factor (as a percent)	2.55%	2.55%
Available-for-Sale Debt Securities and Short Term Investments [Member] \| Recurring [Member] \| Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Percentage of fixed maturity securities valued using model processes to the Company's fixed-income securities and short-term investments at fair value	10.40%