Long-Term Debt and Credit Facilities - Principal and Carrying Amounts of Debt (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	May 18, 2004	Jun. 20, 2014	Dec. 19, 2001
Debt Instrument [Line Items]
Principal	$ 1,596,000,000	$ 1,114,000,000
Carrying Value	1,303,000,000	816,000,000
AGUS [Member]
Debt Instrument [Line Items]
Principal	850,000,000	350,000,000
Carrying Value	847,000,000	348,000,000
AGMH [Member]
Debt Instrument [Line Items]
Principal	730,000,000	730,000,000
Carrying Value	437,000,000	430,000,000
AGM [Member]
Debt Instrument [Line Items]
Principal	16,000,000	34,000,000
Carrying Value	19,000,000	38,000,000
Senior Notes [Member] | AGUS [Member] | Senior Notes 7.0 Percent [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	7.00%	7.00%	7.00%
Principal	200,000,000	200,000,000	200,000,000.0
Carrying Value	198,000,000	198,000,000
Senior Notes [Member] | AGUS [Member] | Senior Notes 5.0 Percent [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	5.00%	0.00%
Principal	500,000,000	0		500,000,000
Carrying Value	499,000,000	0
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 Percent [Member]
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	6.875%	6.875%
Principal	100,000,000	100,000,000			100,000,000.0
Carrying Value	68,000,000	68,000,000
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	139,000,000	138,000,000
Notes Payable, Other Payables [Member] | AGMH [Member] | Notes Payable 5.60 Percent [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	55,000,000	55,000,000
Notes Payable, Other Payables [Member] | AGM [Member]
Debt Instrument [Line Items]
Principal	16,000,000	34,000,000
Carrying Value	19,000,000	38,000,000
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
Carrying Value	$ 175,000,000	$ 169,000,000