Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Basic EPS:
Net income (loss) attributable to AGL	$ 124	$ 389	$ 362
Less: Distributed and undistributed income (loss) available to nonvested shareholders	1	0	1
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 123	$ 389	$ 361
Basic shares (in shares)	62.9	73.5	85.5
Basic EPS (in dollars per share)	$ 1.95	$ 5.29	$ 4.22
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 123	$ 389	$ 361
Plus: Re-allocation of undistributed income (loss) available to nonvested shareholders of AGL and subsidiaries	0	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, diluted	$ 123	$ 389	$ 361
Basic shares (in shares)	62.9	73.5	85.5
Dilutive securities: Options and restricted stock awards (in shares)	1.0	0.8	0.7
Diluted shares (in shares)	63.9	74.3	86.2
Diluted EPS (in dollars per share)	$ 1.92	$ 5.23	$ 4.19
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect	0.6	0.1	0.8