Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic EPS:
Net income (loss) attributable to AGL	$ 69	$ 161	$ 265	$ 433
Less: Distributed and undistributed income (loss) available to nonvested shareholders	0	1	0	1
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 69	$ 160	$ 265	$ 432
Basic shares	98.2	108.0	100.8	111.6
Basic EPS (in dollars per share)	$ 0.71	$ 1.48	$ 2.63	$ 3.87
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 69	$ 160	$ 265	$ 432
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	$ 69	$ 160	$ 265	$ 432
Basic shares	98.2	108.0	100.8	111.6
Effect of dilutive securities:
Options and restricted stock awards (in shares)	0.7	1.3	0.8	1.3
Diluted shares	98.9	109.3	101.6	112.9
Diluted EPS (in dollars per share)	$ 0.70	$ 1.47	$ 2.61	$ 3.83
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.0	0.0	0.0	0.1