Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Basic EPS:
Net income (loss) attributable to AGL	$ 146	$ 297	$ 205	$ 498
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	$ 146	$ 297	$ 205	$ 498
Basic shares	134.0	150.6	135.1	153.2
Basic EPS (in dollars per share)	$ 1.09	$ 1.97	$ 1.52	$ 3.25
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	$ 146	$ 297	$ 205	$ 498
Basic shares	134.0	150.6	135.1	153.2
Effect of dilutive securities:
Options and restricted stock awards (in shares)	0.8	1.0	0.8	0.9
Diluted shares	134.8	151.6	135.9	154.1
Diluted EPS (in dollars per share)	$ 1.09	$ 1.96	$ 1.51	$ 3.23
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect	0.6	0.1	0.7	0.4