Long-Term Debt and Credit Facilities - Principal and Carrying Amounts of Debt (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Principal	$ 1,538,000,000	$ 1,558,000,000
Carrying Value	1,281,000,000	1,292,000,000
Consolidation, Eliminations [Member]
Debt Instrument [Line Items]
Principal	(48,000,000)	(28,000,000)
Carrying Value	(31,000,000)	(18,000,000)
AGUS [Member]
Debt Instrument [Line Items]
Principal	850,000,000	850,000,000
Carrying Value	843,000,000	843,000,000
AGMH [Member]
Debt Instrument [Line Items]
Principal	730,000,000	730,000,000
Carrying Value	463,000,000	461,000,000
AGM [Member]
Debt Instrument [Line Items]
Principal	6,000,000	6,000,000
Carrying Value	$ 6,000,000	$ 6,000,000
Senior Notes [Member] \| AGUS [Member] \| Senior Notes 7.0% [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	7.00%	7.00%
Principal	$ 200,000,000	$ 200,000,000
Carrying Value	$ 197,000,000	$ 197,000,000
Senior Notes [Member] \| AGUS [Member] \| Senior Notes 5.0% [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	5.00%	5.00%
Principal	$ 500,000,000	$ 500,000,000
Carrying Value	496,000,000	496,000,000
Enhanced Junior Subordinated Debentures [Member] \| AGUS [Member]
Debt Instrument [Line Items]
Principal	150,000,000	150,000,000
Carrying Value	$ 150,000,000	$ 150,000,000
Corporate securities [Member] \| AGMH [Member] \| QUIBS 6.875% [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	6.875%	6.875%
Principal	$ 100,000,000	$ 100,000,000
Carrying Value	$ 70,000,000	$ 70,000,000
Notes Payable, Other Payables [Member] \| AGMH [Member] \| 6.25% Notes [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	6.25%	6.25%
Principal	$ 230,000,000	$ 230,000,000
Carrying Value	$ 142,000,000	$ 142,000,000
Notes Payable, Other Payables [Member] \| AGMH [Member] \| Notes Payable 5.60% [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	5.60%	5.60%
Principal	$ 100,000,000	$ 100,000,000
Carrying Value	57,000,000	57,000,000
Notes Payable, Other Payables [Member] \| AGM [Member]
Debt Instrument [Line Items]
Principal	6,000,000	6,000,000
Carrying Value	6,000,000	6,000,000
Junior Subordinated Debt [Member] \| AGMH [Member]
Debt Instrument [Line Items]
Principal	300,000,000	300,000,000
Carrying Value	$ 194,000,000	$ 192,000,000

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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+ References

No definition available.

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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