Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Basic Earnings Per Share (EPS):
Net income (loss) attributable to AGL	$ 81	$ 66
Less: Distributed and undistributed income (loss) available to nonvested shareholders	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 81	$ 66
Basic shares (in shares)	59.1	66.3
Basic EPS (in dollars per share)	$ 1.37	$ 1.00
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 81	$ 66
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	$ 81	$ 66
Basic shares (in shares)	59.1	66.3
Dilutive securities:
Options and restricted stock awards (in shares)	1.3	1.1
Diluted shares (in shares)	60.4	67.4
Diluted EPS (in dollars per share)	$ 1.34	$ 0.98
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.1	0.2