Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic EPS:
Net income (loss) attributable to AGL	$ 197	$ 317
Less: Distributed and undistributed income (loss) available to nonvested shareholders	1	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 196	$ 317
Basic shares	115.2	125.3
Basic EPS (in dollars per share)	$ 1.71	$ 2.53
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	$ 196	$ 317
Basic shares	115.2	125.3
Effect of dilutive securities:
Options and restricted stock awards (in shares)	1.4	1.8
Diluted shares	116.6	127.1
Diluted EPS (in dollars per share)	$ 1.68	$ 2.49
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.2	0.1