Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Basic EPS:
Net income (loss) attributable to AGL	$ 479	$ 129	$ 684	$ 627
Less: Distributed and undistributed income (loss) available to nonvested shareholders	1	0	1	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 478	$ 129	$ 683	$ 627
Basic shares	131.9	145.8	134.0	150.7
Basic EPS (in dollars per share)	$ 3.63	$ 0.88	$ 5.10	$ 4.16
Diluted EPS:
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	$ 478	$ 129	$ 683	$ 627
Basic shares	131.9	145.8	134.0	150.7
Effect of dilutive securities:
Options and restricted stock awards (in shares)	0.9	0.7	0.9	0.9
Diluted shares	132.8	146.5	134.9	151.6
Diluted EPS (in dollars per share)	$ 3.60	$ 0.88	$ 5.06	$ 4.13
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.0	1.3	0.5	0.7