Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Basic EPS:
Net income (loss)	$ 297	$ 159	$ 498	$ 201
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	$ 297	$ 159	$ 498	$ 201
Basic shares	150.6	178.4	153.2	180.3
Basic EPS (in dollars per share)	$ 1.97	$ 0.89	$ 3.25	$ 1.12
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	$ 297	$ 159	$ 498	$ 201
Basic shares	150.6	178.4	153.2	180.3
Effect of dilutive securities:
Options and restricted stock awards (in shares)	1.0	1.1	0.9	1.0
Diluted shares	151.6	179.5	154.1	181.3
Diluted EPS (in dollars per share)	$ 1.96	$ 0.89	$ 3.23	$ 1.11
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect	0.1	1.5	0.4	1.5