Segment Information Segment Information - Reconciliation of Net Income (Loss) Attributable to AGLto Segment Adjusted Operating Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting [Abstract]
Net income (loss) attributable to AGL	$ 183	$ 142	$ 128	$ 196
Net realized investment gains (losses)	4	8	(1)	(4)
Non-credit impairment unrealized fair value gains (losses) on credit derivatives	97	(12)	9	(40)
Fair value gains (losses) on committed capital securities	(25)	19	23	10
Foreign exchange gains (losses) on remeasurement of premiums receivable and loss and LAE reserves	2	(12)	(55)	(3)
Adjustments to Operating Income, Pre-Tax	78	3	(24)	(37)
Adjustments to Operating Income, Tax	(14)	(2)	0	6
Adjusted Income (Loss) Attributable to Parent	$ 119	$ 141	$ 152	$ 227