Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Basic Earnings Per Share (EPS):
Net income (loss) attributable to AGL	$ 17	$ 86	$ 126	$ 214
Less: Distributed and undistributed income (loss) available to nonvested shareholders	0	0	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 17	$ 86	$ 126	$ 214
Basic shares (in shares)	72.7	83.2	74.9	87.4
Basic EPS (in dollars per share)	$ 0.22	$ 1.03	$ 1.67	$ 2.45
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 17	$ 86	$ 126	$ 214
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	$ 17	$ 86	$ 126	$ 214
Basic shares (in shares)	72.7	83.2	74.9	87.4
Dilutive securities:
Options and restricted stock awards (in shares)	0.9	0.6	0.8	0.6
Diluted shares (in shares)	73.6	83.8	75.7	88.0
Diluted EPS (in dollars per share)	$ 0.22	$ 1.02	$ 1.66	$ 2.43
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.0	0.5	0.1	1.0