Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic EPS:
Net income (loss) attributable to AGL	$ 183	$ 142	$ 128	$ 196
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	$ 183	$ 142	$ 128	$ 196
Basic shares	86.5	101.2	89.5	102.1
Basic EPS (in dollars per share)	$ 2.11	$ 1.40	$ 1.43	$ 1.92
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 183	$ 142	$ 128	$ 196
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	$ 183	$ 142	$ 128	$ 196
Basic shares	86.5	101.2	89.5	102.1
Effect of dilutive securities:
Options and restricted stock awards (in shares)	0.5	0.7	0.7	0.9
Diluted shares	87.0	101.9	90.2	103.0
Diluted EPS (in dollars per share)	$ 2.10	$ 1.39	$ 1.42	$ 1.90
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.6	0.1	1.2	0.0