Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Basic Earnings Per Share (EPS):
Net income (loss) attributable to AGL	$ 171	$ 157	$ 358	$ 363
Less: Distributed and undistributed income (loss) available to nonvested shareholders	2	2	3	3
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 169	$ 155	$ 355	$ 360
Basic shares (in shares)	52.4	58.5	54.1	58.9
Basic EPS (in dollars per share)	$ 3.23	$ 2.65	$ 6.57	$ 6.11
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 169	$ 155	$ 355	$ 360
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	$ 169	$ 155	$ 355	$ 360
Basic shares (in shares)	52.4	58.5	54.1	58.9
Dilutive securities:
Restricted stock awards (in shares)	1.0	1.1	1.1	1.1
Diluted shares (in shares)	53.4	59.6	55.2	60.0
Diluted EPS (in dollars per share)	$ 3.17	$ 2.60	$ 6.44	$ 5.99
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.0	0.1	0.1	0.1