Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Basic Earnings Per Share (EPS):
Net income (loss) attributable to AGL	$ 176	$ 109
Less: Distributed and undistributed income (loss) available to nonvested shareholders	1	1
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 175	$ 108
Basic shares (in shares)	50.0	55.6
Basic EPS (in dollars per share)	$ 3.49	$ 1.94
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 175	$ 108
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	$ 175	$ 108
Basic shares (in shares)	50.0	55.6
Dilutive securities:
Restricted stock awards (in shares)	0.7	1.5
Diluted shares (in shares)	50.7	57.1
Diluted EPS (in dollars per share)	$ 3.44	$ 1.89
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.1	0.1