Long-Term Debt and Credit Facilities - Interest Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Interest expense	$ 97	$ 102	$ 101
AGUS [Member]
Debt Instrument [Line Items]
Interest expense	44	48	49
AGMH [Member]
Debt Instrument [Line Items]
Interest expense	54	54	54
AGM [Member]
Debt Instrument [Line Items]
Interest expense	0	0	(2)
Senior Notes [Member] | AGUS [Member] | 7% Senior Notes [Member]
Debt Instrument [Line Items]
Interest expense	13	13	13
Senior Notes [Member] | AGUS [Member] | 5% Senior Notes [Member]
Debt Instrument [Line Items]
Interest expense	26	26	26
Series A Enhanced Junior Subordinated Debentures [Member] | AGUS [Member]
Debt Instrument [Line Items]
Interest expense	5	9	10
Corporate securities [Member] | AGMH [Member] | 6.875% QUIBS [Member]
Debt Instrument [Line Items]
Interest expense	7	7	7
Notes Payable, Other Payables [Member] | AGMH [Member] | 6.25% Notes [Member]
Debt Instrument [Line Items]
Interest expense	16	16	16
Notes Payable, Other Payables [Member] | AGMH [Member] | 5.6% Notes [Member]
Debt Instrument [Line Items]
Interest expense	6	6	6
Notes Payable, Other Payables [Member] | AGM [Member]
Debt Instrument [Line Items]
Interest expense	0	0	(2)
Junior Subordinated Debt [Member] | AGMH [Member]
Debt Instrument [Line Items]
Interest expense	25	25	25
Consolidation, Eliminations [Member]
Debt Instrument [Line Items]
Interest expense	$ (1)	$ 0	$ 0