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, 2020	Mar. 31, 2019
Segment Reporting [Abstract]
Net loss	$ (55)	$ 54
Net realized investment gains (losses)	(5)	(12)
Non-credit impairment unrealized fair value gains (losses) on credit derivatives	(88)	(28)
Fair value gains (losses) on committed capital securities	48	(9)
Foreign exchange gains (losses) on remeasurement of premiums receivable and loss and LAE reserves	(57)	9
Adjustments to Operating Income, Pre-Tax	(102)	(40)
Adjustments to Operating Income, Tax	14	8
Adjusted Income (Loss) Attributable to Parent	$ 33	$ 86