Long-Term Debt and Credit Facilities - Interest Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Interest expense	$ 87	$ 85	$ 89
Senior Notes | AGC | 7% Senior Notes
Debt Instrument [Line Items]
Interest expense	13	13	13
Senior Notes | AGC | 5% Senior Notes
Debt Instrument [Line Items]
Interest expense	23	26	26
Senior Notes | AGC | 3.15% Senior Notes
Debt Instrument [Line Items]
Interest expense	10	0	0
Senior Notes | AGC | 3.6% Senior Notes
Debt Instrument [Line Items]
Interest expense	5	0	0
Enhanced Junior Subordinated Debentures | AGC
Debt Instrument [Line Items]
Interest expense	4	5	7
Corporate securities | AGMH | 6.875% QUIBS
Debt Instrument [Line Items]
Interest expense	4	7	7
Notes Payable, Other Payables | AGMH | 6.25% Notes
Debt Instrument [Line Items]
Interest expense	10	15	16
Notes Payable, Other Payables | AGMH | 5.6% Notes
Debt Instrument [Line Items]
Interest expense	5	6	6
Junior Subordinated Debentures | AGMH
Debt Instrument [Line Items]
Interest expense	12	13	14
Other | Other
Debt Instrument [Line Items]
Interest expense	$ 1	$ 0	$ 0