Long-Term Debt - Principal Carrying Amount of Debt (Details) - USD ($)	Jun. 30, 2021	May 21, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Principal	$ (1,928,000,000)		$ (1,429,000,000)
Carrying Value	(1,720,000,000)		(1,224,000,000)
AGUS
Debt Instrument [Line Items]
Principal	(1,630,000,000)		(1,130,000,000)
Carrying Value	(1,620,000,000)		(1,125,000,000)
AGMH
Debt Instrument [Line Items]
Principal	(730,000,000)		(730,000,000)
Carrying Value	(487,000,000)		(483,000,000)
AGM
Debt Instrument [Line Items]
Principal	(2,000,000)		(3,000,000)
Carrying Value	$ (2,000,000)		(3,000,000)
Senior Notes | AGUS | 7% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	7.00%
Principal	$ (200,000,000)		(200,000,000)
Carrying Value	$ (197,000,000)		(197,000,000)
Senior Notes | AGUS | 5% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	5.00%
Principal	$ (500,000,000)		(500,000,000)
Carrying Value	$ (499,000,000)		(498,000,000)
Senior Notes | AGUS | 3.15% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	3.15%	3.15%
Principal	$ (500,000,000)	$ (500,000,000)	0
Carrying Value	(494,000,000)		0
Basis points		0.25%
Enhanced Junior Subordinated Debentures | AGUS
Debt Instrument [Line Items]
Principal	(150,000,000)		(150,000,000)
Carrying Value	(150,000,000)		(150,000,000)
AGUS - long-term debt | AGUS
Debt Instrument [Line Items]
Principal	(1,350,000,000)		(850,000,000)
Carrying Value	(1,340,000,000)		(845,000,000)
Intercompany Loans Payable | AGUS
Debt Instrument [Line Items]
Principal	(280,000,000)		(280,000,000)
Carrying Value	$ (280,000,000)		(280,000,000)
Corporate securities | AGMH | 6.875% QUIBS
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	6.875%
Principal	$ (100,000,000)		(100,000,000)
Carrying Value	$ (71,000,000)		(71,000,000)
Notes Payable, Other Payables | AGMH | 6.25% Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	6.25%
Principal	$ (230,000,000)		(230,000,000)
Carrying Value	$ (146,000,000)		(145,000,000)
Notes Payable, Other Payables | AGMH | 5.6% Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	5.60%
Principal	$ (100,000,000)		(100,000,000)
Carrying Value	(58,000,000)		(58,000,000)
Notes Payable, Other Payables | AGM
Debt Instrument [Line Items]
Principal	(2,000,000)		(3,000,000)
Carrying Value	(2,000,000)		(3,000,000)
Junior Subordinated Debentures | AGMH
Debt Instrument [Line Items]
Principal	(300,000,000)		(300,000,000)
Carrying Value	(212,000,000)		(209,000,000)
Consolidation, Eliminations | AGMH's Debt Purchased by AGUS | AGUS
Debt Instrument [Line Items]
Principal	(154,000,000)		154,000,000
Carrying Value	$ 109,000,000		$ 107,000,000