Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Basic Earnings Per Share (EPS):
Net income (loss) attributable to AGL	$ 109	$ 81
Less: Distributed and undistributed income (loss) available to nonvested shareholders	1	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 108	$ 81
Basic shares (in shares)	55.6	59.1
Basic EPS (in dollars per share)	$ 1.94	$ 1.37
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 108	$ 81
Plus: Re-allocation of undistributed income (loss) available to nonvested shareholders of AGL and subsidiaries	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, diluted	$ 108	$ 81
Basic shares (in shares)	55.6	59.1
Dilutive securities:
Restricted stock awards (in shares)	1.5	1.3
Diluted shares (in shares)	57.1	60.4
Diluted EPS (in dollars per share)	$ 1.89	$ 1.34
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.1	0.1