Long-Term Debt and Credit Facilities - Principal and Carrying Amounts of Debt (Details) (USD $)	Sep. 30, 2014	Jun. 20, 2014	Dec. 31, 2013
Debt
Principal	$ 1,597,000,000		$ 1,114,000,000
Carrying Value	1,303,000,000		816,000,000
AGUS Consolidated
Debt
Interest rate of debt (as a percent)			0.00%
Principal	850,000,000		350,000,000
Carrying Value	847,000,000		348,000,000
AGUS Consolidated | 7.0% Senior Notes
Debt
Interest rate of debt (as a percent)	7.00%		7.00%
Principal	200,000,000		200,000,000
Carrying Value	198,000,000		198,000,000
AGUS Consolidated | 5.0% Senior Notes
Debt
Interest rate of debt (as a percent)	5.00%	5.00%
Principal	500,000,000	500,000,000	0
Carrying Value	499,000,000		0
AGUS Consolidated | Series A Enhanced Junior Subordinated Debentures
Debt
Principal	150,000,000		150,000,000
Carrying Value	150,000,000		150,000,000
AGMH Consolidated
Debt
Principal	730,000,000		730,000,000
Carrying Value	436,000,000		430,000,000
AGMH Consolidated | 6 7/8% QUIBS
Debt
Interest rate of debt (as a percent)	6.875%		6.875%
Principal	100,000,000		100,000,000
Carrying Value	68,000,000		68,000,000
AGMH Consolidated | 6.25% Notes
Debt
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
AGMH Consolidated | 5.60% Notes
Debt
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
AGMH Consolidated | Junior Subordinated Debentures
Debt
Principal	300,000,000		300,000,000
Carrying Value	174,000,000		169,000,000
AGM | AGM Notes Payable
Debt
Principal	17,000,000		34,000,000
Carrying Value	$ 20,000,000		$ 38,000,000