Long-Term Debt and Credit Facilities - Principal and Carrying Amounts of Debt (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Aug. 21, 2023	Aug. 20, 2021	May 26, 2021	Nov. 22, 2006	May 18, 2004
Debt Instrument [Line Items]
Principal	$ 1,746	$ 1,746
Carrying Value	$ 1,699	1,694
Senior Notes | AGC | AGUS 6.125% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	6.125%		6.125%
Principal	$ 350	350	$ 350
Carrying Value	$ 346	345
Senior Notes | AGC | AGUS 3.15% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	3.15%
Principal	$ 500	500			$ 500
Carrying Value	$ 496	496
Senior Notes | AGC | AGUS 7% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	700.00%						700.00%
Principal	$ 200	200					$ 200
Carrying Value	$ 198	198
Senior Notes | AGC | AGUS 3.6% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	3.60%
Principal	$ 400	400		$ 400
Carrying Value	396	395
Senior Notes | AGMH | AGUS 3.15% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)					3.15%
Senior Notes | AGMH | AGUS 3.6% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)				3.60%
Enhanced Junior Subordinated Debentures | AGC
Debt Instrument [Line Items]
Principal	150	150
Carrying Value	150	150
Junior Subordinated Debentures | AGMH
Debt Instrument [Line Items]
Principal	146	146				$ 300
Carrying Value	$ 113	$ 110