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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting [Abstract]
Net income (loss) attributable to AGL	$ 86	$ 69	$ 214	$ 265
Realized gains (losses) on investments	13	16	12	12
Non-credit impairment unrealized fair value gains (losses) on credit derivatives	(3)	11	6	(29)
Fair value gains (losses) on CCS (1)	(10)	(14)	13	(4)
Foreign exchange gains (losses) on remeasurement of premiums receivable and loss and LAE reserves	40	(20)	(15)	(23)
Total pre-tax adjustments	40	(7)	16	(44)
Less tax effect on pre-tax adjustments	(2)	(1)	(2)	5
Adjusted operating income (loss)	$ 48	$ 77	$ 200	$ 304