Segment Information - Reconciliation of Net Income (Loss) Attributable to AGL to Segment Adjusted Operating Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Revenues	$ 345	$ 245
Expenses	169	125
Equity in Earnings (Losses) of Investees	53	24
Less: Provision (benefit) for income taxes	44	31
Less: Noncontrolling interests	9	4
Net Income (Loss) Attributable to AGL	176	109
Realized gains (losses) on investments
Segment Reporting Information [Line Items]
Revenues	(16)	8
Net Income (Loss) Attributable to AGL	(16)	8
Non-credit impairment-related unrealized fair value gains (losses) on credit derivatives
Segment Reporting Information [Line Items]
Revenues	61	8
Net Income (Loss) Attributable to AGL	(2)	10
Fair value gains (losses) on CCS
Segment Reporting Information [Line Items]
Revenues	2	(10)
Net Income (Loss) Attributable to AGL	2	(10)
Foreign exchange gains (losses) on remeasurement of premiums receivable and loss and LAE reserves
Segment Reporting Information [Line Items]
Revenues	33	(12)
Net Income (Loss) Attributable to AGL	33	(12)
Tax effect
Segment Reporting Information [Line Items]
Less: Provision (benefit) for income taxes	3
Net Income (Loss) Attributable to AGL	(3)	0
Subtotal
Segment Reporting Information [Line Items]
Revenues		251
Expenses		127
Equity in Earnings (Losses) of Investees		24
Less: Provision (benefit) for income taxes		31
Less: Noncontrolling interests		4
Net Income (Loss) Attributable to AGL		113
Subtotal
Segment Reporting Information [Line Items]
Revenues	265
Expenses	106
Equity in Earnings (Losses) of Investees	53
Less: Provision (benefit) for income taxes	41
Less: Noncontrolling interests	9
Net Income (Loss) Attributable to AGL	162
Operating Segments
Segment Reporting Information [Line Items]
Revenues	245	230
Expenses	68	85
Equity in Earnings (Losses) of Investees	43	41
Less: Provision (benefit) for income taxes	40	36
Less: Noncontrolling interests	0	0
Net Income (Loss) Attributable to AGL	180	150
Operating Segments | Insurance
Segment Reporting Information [Line Items]
Revenues	239	229
Expenses	64	85
Equity in Earnings (Losses) of Investees	30	40
Less: Provision (benefit) for income taxes	37	35
Less: Noncontrolling interests	0	0
Net Income (Loss) Attributable to AGL	168	149
Operating Segments | Asset Management
Segment Reporting Information [Line Items]
Revenues	6	1
Expenses	4	0
Equity in Earnings (Losses) of Investees	13	1
Less: Provision (benefit) for income taxes	3	1
Less: Noncontrolling interests	0	0
Net Income (Loss) Attributable to AGL	12	1
Corporate division
Segment Reporting Information [Line Items]
Revenues	4	5
Expenses	40	47
Equity in Earnings (Losses) of Investees	16	0
Less: Provision (benefit) for income taxes	0	(5)
Less: Noncontrolling interests	0	0
Net Income (Loss) Attributable to AGL	(20)	(37)
Reconciling items:
Segment Reporting Information [Line Items]
Expenses	63	(2)
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	16	16
Expenses	(2)	(5)
Equity in Earnings (Losses) of Investees	(6)	(17)
Less: Provision (benefit) for income taxes	1	0
Less: Noncontrolling interests	9	4
Net Income (Loss) Attributable to AGL	2	0
Intersegment Eliminations | Insurance
Segment Reporting Information [Line Items]
Revenues	2	2
Intersegment Eliminations | Asset Management
Segment Reporting Information [Line Items]
Revenues	$ 1	$ 0