Segment Information Segment Information - Reconciliation of Net Income (Loss) Attributable to AGLto Segment Adjusted Operating Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting [Abstract]
Net income (loss) attributable to AGL	$ 11	$ (55)
Realized gains (losses) on investments	(3)	(5)
Non-credit impairment unrealized fair value gains (losses) on credit derivatives	(19)	(88)
Fair value gains (losses) on CCS	(19)	48
Foreign exchange gains (losses) on remeasurement of premiums receivable and loss and LAE reserves	1	(57)
Total pre-tax adjustments	(40)	(102)
Less tax effect on pre-tax adjustments	8	14
Adjusted operating income (loss)	$ 43	$ 33