Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Basic EPS:
Net income (loss) attributable to AGL	$ 317	$ 59
Less: Distributed and undistributed income (loss) available to nonvested shareholders	0	1
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 317	$ 58
Basic shares	125.3	136.2
Basic EPS (in dollars per share)	$ 2.53	$ 0.43
Diluted EPS:
Plus: Re-allocation of undistributed income (loss) available to nonvested shareholders of AGL and subsidiaries	$ 0	$ 0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, diluted	$ 317	$ 58
Basic shares	125.3	136.2
Effect of dilutive securities:
Options and restricted stock awards (in shares)	1.8	0.8
Diluted shares	127.1	137.0
Diluted EPS (in dollars per share)	$ 2.49	$ 0.43
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.1	0.8