Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Basic EPS:
Net income (loss)	$ 129	$ 355	$ 627	$ 556
Less: Distributed and undistributed income (loss) available to nonvested shareholders	0	1	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 129	$ 354	$ 627	$ 556
Basic shares	145.8	168.8	150.7	176.4
Basic EPS (in dollars per share)	$ 0.88	$ 2.10	$ 4.16	$ 3.15
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	$ 129	$ 354	$ 627	$ 556
Basic shares	145.8	168.8	150.7	176.4
Effect of dilutive securities:
Options and restricted stock awards (in shares)	0.7	0.9	0.9	1.0
Diluted shares	146.5	169.7	151.6	177.4
Diluted EPS (in dollars per share)	$ 0.88	$ 2.09	$ 4.13	$ 3.13
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect	1.3	1.9	0.7	1.6