Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Basic EPS:
Net income (loss)	$ 532		$ 355		$ 159		$ 42		$ 349		$ 384		$ 219		$ (144)		$ 1,088		$ 808		$ 110
Less: Distributed and undistributed income (loss) available to nonvested shareholders																	0		1		0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic																	1,088		807		110
Basic shares																	172.6		186.6		189.2
Basic EPS (in dollars per share)	$ 3.30	[1]	$ 2.10	[1]	$ 0.89	[1]	$ 0.23	[1]	$ 1.91	[1]	$ 2.10	[1]	$ 1.17	[1]	$ (0.74)	[1]	$ 6.30	[1]	$ 4.32	[1]	$ 0.58
Diluted EPS:
Plus: Re-allocation of undistributed income (loss) available to nonvested shareholders of AGL and subsidiaries																	0		0		0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, diluted																	$ 1,088		$ 807		$ 110
Basic shares																	172.6		186.6		189.2
Effect of dilutive securities:
Options and restricted stock awards (in shares)																	1.0		1.0		0.8
Equity units (in shares)																	0		0		0.7
Diluted shares																	173.6		187.6		190.7
Diluted EPS (in dollars per share)	$ 3.28	[1]	$ 2.09	[1]	$ 0.89	[1]	$ 0.23	[1]	$ 1.90	[1]	$ 2.09	[1]	$ 1.16	[1]	$ (0.74)	[1]	$ 6.26	[1]	$ 4.30	[1]	$ 0.57
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect																	1.6		2.7		9.9

[1] Per share amounts for the quarters and the full years have each been calculated separately. Accordingly, quarterly amounts may not sum up to the annual amounts because of differences in the average common shares outstanding during each period and, with regard to diluted per share amounts only, because of the inclusion of the effect of potentially dilutive securities only in the periods in which such effect would have been dilutive.