Segment Information Segment Information - Reconciliation of Net Income (Loss) Attributable to AGLto Segment Adjusted Operating Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting [Abstract]
Net income (loss) attributable to AGL	$ 17	$ 86	$ 126	$ 214
Less pre-tax adjustments:
Realized gains (losses) on investments	3	13	4	12
Non-credit impairment unrealized fair value gains (losses) on credit derivatives	9	(3)	(41)	6
Fair value gains (losses) on CCS	(3)	(10)	(28)	13
Foreign exchange gains (losses) on remeasurement of premiums receivable and loss and LAE reserves	(27)	40	(21)	(15)
Total pre-tax adjustments	(18)	40	(86)	16
Less tax effect on pre-tax adjustments	1	(2)	15	(2)
Adjusted operating income (loss)	$ 34	$ 48	$ 197	$ 200