Long-Term Debt and Credit Facilities - Principal and Carrying Amounts of Debt (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	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	$ 1,589,000,000	$ 1,592,000,000
Carrying Value	1,306,000,000	1,300,000,000
AGUS [Member]
Debt Instrument [Line Items]
Principal	850,000,000	850,000,000
Carrying Value	843,000,000	842,000,000
AGMH [Member]
Debt Instrument [Line Items]
Principal	730,000,000	730,000,000
Carrying Value	453,000,000	445,000,000
AGM [Member]
Debt Instrument [Line Items]
Principal	9,000,000	12,000,000
Carrying Value	$ 10,000,000	13,000,000
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	496,000,000	495,000,000
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	$ 69,000,000	69,000,000
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	$ 141,000,000	140,000,000
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	56,000,000	56,000,000
Notes Payable, Other Payables [Member] | AGM [Member]
Debt Instrument [Line Items]
Principal	9,000,000	12,000,000
Carrying Value	10,000,000	13,000,000
Junior Subordinated Debt [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	$ 187,000,000	$ 180,000,000