Long-Term Debt and Credit Facilities - Principal and Carrying Amounts of Debt (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Jun. 20, 2014	Dec. 20, 2006	Nov. 22, 2006	May 18, 2004	Jul. 31, 2003	Nov. 26, 2002	Dec. 19, 2001
Debt Instrument [Line Items]
Principal	$ 1,457,000,000	$ 1,558,000,000
Carrying Value	1,233,000,000	1,292,000,000
AGUS [Member]
Debt Instrument [Line Items]
Principal	850,000,000	850,000,000
Carrying Value	844,000,000	843,000,000
AGMH [Member]
Debt Instrument [Line Items]
Principal	730,000,000	730,000,000
Carrying Value	468,000,000	461,000,000
AGM [Member]
Debt Instrument [Line Items]
Principal	5,000,000	6,000,000
Carrying Value	$ 5,000,000	6,000,000
Senior Notes [Member] | AGUS [Member] | 7% Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	7.00%
Principal	$ 200,000,000	200,000,000				$ 200,000,000.0
Carrying Value	$ 197,000,000	197,000,000
Senior Notes [Member] | AGUS [Member] | 5% Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	5.00%
Principal	$ 500,000,000	500,000,000	$ 500,000,000
Carrying Value	497,000,000	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] | 6.875% QUIBS [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	6.875%
Principal	$ 100,000,000	100,000,000							$ 100,000,000.0
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%
Principal	$ 230,000,000	230,000,000						$ 230,000,000.0
Carrying Value	$ 143,000,000	142,000,000
Notes Payable, Other Payables [Member] | AGMH [Member] | 5.6% Notes [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	5.60%
Principal	$ 100,000,000	100,000,000					$ 100,000,000.0
Carrying Value	57,000,000	57,000,000
Notes Payable, Other Payables [Member] | AGM [Member]
Debt Instrument [Line Items]
Principal	5,000,000	6,000,000
Carrying Value	5,000,000	6,000,000
Junior Subordinated Debentures [Member] | AGMH [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)				6.40%
Principal	300,000,000	300,000,000			$ 300,000,000
Carrying Value	198,000,000	192,000,000
Consolidating Adjustments [Member]
Debt Instrument [Line Items]
Principal	(128,000,000)	(28,000,000)
Carrying Value	$ (84,000,000)	$ (18,000,000)