Long-Term Debt and Credit Facilities - Principal and Carrying Amounts of Debt (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Principal	$ 1,454	$ 1,457
Carrying Value	1,232	1,233
Consolidation, Eliminations [Member]
Debt Instrument [Line Items]
Principal	(131)	(128)
Carrying Value	(87)	(84)
AGUS [Member]
Debt Instrument [Line Items]
Principal	850	850
Carrying Value	844	844
AGMH [Member]
Debt Instrument [Line Items]
Principal	730	730
Carrying Value	470	468
AGM [Member]
Debt Instrument [Line Items]
Principal	5	5
Carrying Value	$ 5	$ 5
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	$ 200
Carrying Value	$ 197	$ 197
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	$ 500
Carrying Value	497	497
Enhanced Junior Subordinated Debentures [Member] \| AGUS [Member]
Debt Instrument [Line Items]
Principal	150	150
Carrying Value	$ 150	$ 150
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	$ 100
Carrying Value	$ 70	$ 70
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	$ 230
Carrying Value	$ 144	$ 143
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	$ 100
Carrying Value	57	57
Notes Payable, Other Payables [Member] \| AGM [Member]
Debt Instrument [Line Items]
Principal	5	5
Carrying Value	5	5
Junior Subordinated Debt [Member] \| AGMH [Member]
Debt Instrument [Line Items]
Principal	300	300
Carrying Value	$ 199	$ 198

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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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.

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