Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic EPS:
Net income (loss) attributable to AGL	$ 142	$ 75	$ 196	$ 272
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	$ 142	$ 75	$ 196	$ 272
Basic shares	101.2	111.7	102.1	113.4
Basic EPS (in dollars per share)	$ 1.40	$ 0.67	$ 1.92	$ 2.39
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 142	$ 75	$ 196	$ 272
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	$ 142	$ 75	$ 196	$ 272
Basic shares	101.2	111.7	102.1	113.4
Effect of dilutive securities:
Options and restricted stock awards (in shares)	0.7	1.2	0.9	1.4
Diluted shares	101.9	112.9	103.0	114.8
Diluted EPS (in dollars per share)	$ 1.39	$ 0.67	$ 1.90	$ 2.37
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.1	0.1	0.0	0.2