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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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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.

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Amount that would be paid, determined under the conditions specified in the contract, if the holder of the share has the right to redeem the shares.

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Long-Term Debt and Credit Facilities - Narrative (Details)					1 Months Ended	12 Months Ended
	Jun. 20, 2014 USD ($)	Apr. 07, 2008	Dec. 20, 2006 USD ($) period	May 18, 2004 USD ($)	Jun. 30, 2003	Dec. 31, 2018 USD ($) extension	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 25, 2013 USD ($)	Dec. 31, 2012 USD ($)	Nov. 22, 2006 USD ($)	Apr. 08, 2005 USD ($) Trust	Jul. 31, 2003 USD ($)	Nov. 26, 2002 USD ($)	Dec. 19, 2001 USD ($)
Debt Instrument [Line Items]
Aggregate principal amount						$ 1,457,000,000	$ 1,558,000,000
AGC Trust Preferred Securities [Member]
Debt Instrument [Line Items]
Basis points		2.50%
AGM Trust Preferred Securities [Member]
Debt Instrument [Line Items]
Basis points					2.00%
AGUS [Member]
Debt Instrument [Line Items]
Aggregate principal amount						850,000,000	850,000,000
AGM and AGC [Member] \| AGM CPS securities [Member]
Debt Instrument [Line Items]
Maximum stock purchase obligation of each custodial trust												$ 200,000,000
Number of custodial trusts \| Trust												4
Aggregate maximum stock purchase obligation of the custodial trusts												$ 50,000,000.0
AGM [Member]
Debt Instrument [Line Items]
Aggregate principal amount						5,000,000	6,000,000
AGM [Member] \| AGM CPS securities [Member]
Debt Instrument [Line Items]
Rate basis for income distributions					one-month LIBOR
AGC [Member] \| AGM CPS securities [Member]
Debt Instrument [Line Items]
Rate basis for income distributions		one-month LIBOR
AGMH [Member]
Debt Instrument [Line Items]
Aggregate principal amount						730,000,000	730,000,000
Junior Subordinated Debt [Member] \| AGMH [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)			6.40%
Aggregate principal amount						$ 300,000,000	300,000,000				$ 300,000,000
Interest rate, added to base rate (as a percent)						2.215%
Number of times repayment date may be extended \| extension						4
Junior Subordinated Debt [Member] \| Maximum [Member] \| AGMH [Member]
Debt Instrument [Line Items]
Period for increments of repayment date extension						5 years
Line of Credit [Member] \| Revolving Credit Facility [Member] \| Wilbur L. Ross [Member]
Debt Instrument [Line Items]
Interest rate, as a percentage of Federal short-term or mid-term interest rate									100.00%
Notes Payable, Other Payables [Member] \| AGM [Member]
Debt Instrument [Line Items]
Aggregate principal amount						$ 5,000,000	6,000,000
5.6% Notes [Member] \| Notes Payable, Other Payables [Member] \| AGMH [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)						5.60%
Aggregate principal amount						$ 100,000,000	100,000,000						$ 100,000,000.0
6.25% Notes [Member] \| Notes Payable, Other Payables [Member] \| AGMH [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)						6.25%
Aggregate principal amount						$ 230,000,000	230,000,000							$ 230,000,000.0
6.875% QUIBS [Member] \| Corporate securities [Member] \| AGMH [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)						6.875%
Aggregate principal amount						$ 100,000,000	100,000,000								$ 100,000,000.0
Enhanced Junior Subordinated Debentures [Member] \| Junior Subordinated Debt [Member] \| AGUS [Member]
Debt Instrument [Line Items]
Aggregate principal amount			$ 150,000,000
Interest rate, added to base rate (as a percent)			2.38%
Enhanced Junior Subordinated Debentures [Member] \| Junior Subordinated Debt [Member] \| AGMH [Member]
Debt Instrument [Line Items]
Number of times interest payment may be deferred			1
Number of consecutive periods for which interest payments may be deferred \| period			1
Period for which interest payment may be deferred (in years)			10 years
Period prior to final repayment date before which debt cannot be repaid, redeemed, repurchased or defeased (in years)						20 years
7% Senior Notes [Member] \| Senior Notes [Member] \| AGUS [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)						7.00%
Aggregate principal amount				$ 200,000,000.0		$ 200,000,000	200,000,000
Net proceeds from issuance of debt				$ 197,000,000
Effective interest rate of debt (as a percent)						6.40%
5% Senior Notes [Member] \| Senior Notes [Member] \| AGUS [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)						5.00%
Aggregate principal amount	$ 500,000,000					$ 500,000,000	500,000,000
Net proceeds from issuance of debt	$ 495,000,000
Intercompany Credit Facility [Member] \| Line of Credit [Member] \| Revolving Credit Facility [Member] \| Wilbur L. Ross [Member] \| Assured Guaranty LTD [Member]
Debt Instrument [Line Items]
Related party, maximum borrowing capacity									$ 225,000,000
Amounts currently outstanding						0
MAC [Member] \| AGUS [Member]
Debt Instrument [Line Items]
Repayments of Debt						10,000,000	$ 10,000,000	$ 20,000,000
Long-term Debt						$ 50,000,000				$ 90,000,000