Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Basic Earnings Per Share (EPS):
Net income (loss) attributable to AGL	$ 98	$ 183	$ 109	$ 128
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	$ 98	$ 183	$ 109	$ 128
Basic shares (in shares)	75.2	86.5	75.9	89.5
Basic EPS (in dollars per share)	$ 1.31	$ 2.11	$ 1.44	$ 1.43
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 98	$ 183	$ 109	$ 128
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	$ 98	$ 183	$ 109	$ 128
Basic shares (in shares)	75.2	86.5	75.9	89.5
Dilutive securities:
Options and restricted stock awards (in shares)	0.8	0.5	0.8	0.7
Diluted shares (in shares)	76.0	87.0	76.7	90.2
Diluted EPS (in dollars per share)	$ 1.29	$ 2.10	$ 1.42	$ 1.42
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.0	0.6	0.2	1.2