Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Basic Earnings Per Share (EPS):
Net income (loss) attributable to AGL	$ 125	$ (47)	$ 206	$ 19
Less: Distributed and undistributed income (loss) available to nonvested shareholders	1	0	1	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 124	$ (47)	$ 205	$ 19
Basic shares (in shares)	59.2	63.8	59.1	65.0
Basic EPS (in dollars per share)	$ 2.09	$ (0.74)	$ 3.46	$ 0.29
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 124	$ (47)	$ 205	$ 19
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	$ 124	$ (47)	$ 205	$ 19
Basic shares (in shares)	59.2	63.8	59.1	65.0
Dilutive securities:
Options and restricted stock awards (in shares)	0.9	0.0	1.2	1.2
Diluted shares (in shares)	60.1	63.8	60.3	66.2
Diluted EPS (in dollars per share)	$ 2.06	$ (0.74)	$ 3.40	$ 0.29
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.2	2.1	0.1	1.2