Earnings Per Share (Details) (USD $) Share data in Millions, except Per Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Basic earnings per share:
Net income (loss)	$ 761,153,000	$ 164,599,000	$ 859,229,000	$ 677,194,000
Less: Distributed and undistributed income (loss) available to nonvested shareholders	400,000	300,000	500,000	1,300,000
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	760,800,000	164,300,000	858,700,000	675,900,000
Basic shares (in shares)	183.2	183.7	183.7	184.0
Basic earnings per share (in dollars per share)	$ 4.15	$ 0.89	$ 4.67	$ 3.67
Diluted earnings per share:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	760,800,000	164,300,000	858,700,000	675,900,000
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	$ 760,800,000	$ 164,300,000	$ 858,700,000	$ 675,900,000
Basic shares (in shares)	183.2	183.7	183.7	184.0
Effect of dilutive securities:
Options and restricted stock awards (in shares)	0.8	0.8	0.9	0.9
Equity units (in shares)		2.8	1.7	4.0
Diluted shares (in shares)	184.0	187.3	186.3	188.9
Diluted earnings per share (in dollars per share)	$ 4.13	$ 0.88	$ 4.61	$ 3.58
Potentially dilutive securities excluded from computation of earnings per share because of antidilutive effect (in shares)	3.5	4.7	3.4	3.2