Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Basic Earnings Per Share (EPS):
Net income (loss) attributable to AGL	$ 86	$ 69	$ 214	$ 265
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	$ 86	$ 69	$ 214	$ 265
Basic shares (in shares)	83.2	98.2	87.4	100.8
Basic EPS (in dollars per share)	$ 1.03	$ 0.71	$ 2.45	$ 2.63
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 86	$ 69	$ 214	$ 265
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	$ 86	$ 69	$ 214	$ 265
Basic shares (in shares)	83.2	98.2	87.4	100.8
Dilutive securities:
Options and restricted stock awards (in shares)	0.6	0.7	0.6	0.8
Diluted shares (in shares)	83.8	98.9	88.0	101.6
Diluted EPS (in dollars per share)	$ 1.02	$ 0.70	$ 2.43	$ 2.61
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.5	0.0	1.0	0.0