Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Basic Earnings Per Share (EPS):
Net income (loss) attributable to AGL	$ 11	$ 17	$ 30	$ 126
Less: Distributed and undistributed income (loss) available to nonvested shareholders	0	0	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 11	$ 17	$ 30	$ 126
Basic shares (in shares)	61.7	72.7	63.9	74.9
Basic EPS (in dollars per share)	$ 0.18	$ 0.22	$ 0.47	$ 1.67
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 11	$ 17	$ 30	$ 126
Plus: Re-allocation of undistributed income (loss) available to nonvested shareholders of AGL and subsidiaries	0	0	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, diluted	$ 11	$ 17	$ 30	$ 126
Basic shares (in shares)	61.7	72.7	63.9	74.9
Dilutive securities:
Options and restricted stock awards (in shares)	1.2	0.9	1.2	0.8
Diluted shares (in shares)	62.9	73.6	65.1	75.7
Diluted EPS (in dollars per share)	$ 0.18	$ 0.22	$ 0.46	$ 1.66
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.1	0.0	0.8	0.1