Earnings Per Share (Details) (USD $) Share data in Millions, except Per Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Basic earnings per share:
Net income (loss)	$ (42,536,000)	$ 179,135,000	$ 98,076,000	$ 512,595,000
Less: Distributed and undistributed income (loss) available to nonvested shareholders		300,000	100,000	1,000,000
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	(42,600,000)	178,800,000	97,900,000	511,600,000
Basic shares (in shares)	184.2	184.1	184.0	184.2
Basic earnings per share (in dollars per share)	$ (0.23)	$ 0.97	$ 0.53	$ 2.78
Diluted earnings per share:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	(42,600,000)	178,800,000	97,900,000	511,600,000
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	$ (42,600,000)	$ 178,800,000	$ 97,900,000	$ 511,600,000
Basic shares (in shares)	184.2	184.1	184.0	184.2
Effect of dilutive securities:
Options and restricted stock awards (in shares)		0.8	0.9	0.9
Equity units (in shares)		3.9	2.5	4.7
Diluted shares (in shares)	184.2	188.8	187.4	189.8
Diluted earnings per share (in dollars per share)	$ (0.23)	$ 0.95	$ 0.52	$ 2.70
Potentially dilutive securities excluded from computation of earnings per share because of antidilutive effect (in shares)	9.6	2.7	2.9	2.4