Long-Term Debt - Principal Carrying Amount of Debt (Details) - USD ($) $ in Millions	Sep. 30, 2023	Aug. 21, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 1,746		$ 1,726
Long-term Debt	$ 1,693		1,675
Senior Notes | AGUS | 5% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	5.00%
Debt Instrument, Face Amount	$ 0		330
Long-term Debt	$ 0		329
Senior Notes | AGUS | 6.125% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	6.125%	6.125%
Debt Instrument, Face Amount	$ 350	$ 350	0
Long-term Debt	$ 345		0
Senior Notes | AGUS | 3.15% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	3.15%
Debt Instrument, Face Amount	$ 500		500
Long-term Debt	$ 495		495
Senior Notes | AGUS | 7% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	700.00%
Debt Instrument, Face Amount	$ 200		200
Long-term Debt	$ 198		198
Senior Notes | AGUS | 3.6% Senior Notes
Debt Instrument [Line Items]
Interest rate of debt (as a percent)	3.60%
Debt Instrument, Face Amount	$ 400		400
Long-term Debt	395		395
Enhanced Junior Subordinated Debentures | AGUS
Debt Instrument [Line Items]
Debt Instrument, Face Amount	150		150
Long-term Debt	150		150
Junior Subordinated Debentures | AGMH
Debt Instrument [Line Items]
Debt Instrument, Face Amount	146		146
Long-term Debt	$ 110		$ 108