Long-Term Debt and Credit Facilities - Interest Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Interest expense	$ 94	$ 97	$ 102
AGUS [Member]
Debt Instrument [Line Items]
Interest expense	46	44	48
AGMH [Member]
Debt Instrument [Line Items]
Interest expense	53	54	54
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	7	5	9
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	15	16	16
Notes Payable, Other Payables [Member] | AGMH [Member] | 5.6% Notes [Member]
Debt Instrument [Line Items]
Interest expense	6	6	6
Junior Subordinated Debentures [Member] | AGMH [Member]
Debt Instrument [Line Items]
Interest expense	25	25	25
Purchased debt [Member]
Debt Instrument [Line Items]
Interest expense	$ (5)	$ (1)	$ 0