Long-Term Debt and Credit Facilities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Mar. 31, 2013
Debt
Principal	$ 1,141	$ 1,135
Carrying Value	836	832
AGUS Consolidated
Debt
Principal	350	350
Carrying Value	347	348
AGUS Consolidated | 7.0% Senior Notes [Member]
Debt
Principal	200	200
Carrying Value	197	198
Interest rate of debt (as a percent)	7.00%	7.00%
AGUS Consolidated | Enhanced Junior Subordinated Debentures [Member]
Debt
Interest rate, descripton	3 month LIBOR
Principal	150	150
Carrying Value	150	150
AGMH Consolidated
Debt
Principal	730	730
Carrying Value	423	424
AGMH Consolidated | 6 7/8% QUIBS [Member]
Debt
Principal	100	100
Carrying Value	68	68
Interest rate of debt (as a percent)	6.875%	6.875%
AGMH Consolidated | 6.25% Notes [Member]
Debt
Principal	230	230
Carrying Value	137	137
Interest rate of debt (as a percent)	6.25%	6.25%
AGMH Consolidated | 5.60% Notes [Member]
Debt
Principal	100	100
Carrying Value	54	54
Interest rate of debt (as a percent)	5.60%	5.60%
AGMH Consolidated | Junior Subordinated Debentures [Member]
Debt
Interest rate, descripton	One month LIBOR
Principal	300	300
Carrying Value	164	165
AGM
Debt
Principal	61	55
Carrying Value	66	60
AGM | Notes Payable
Debt
Principal	61	55
Carrying Value	$ 66	$ 60