Earnings Per Share (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Basic earnings per share ("EPS"):
Net income (loss)	$ (483,024,000)	$ 139,256,000	$ 0
Less: Distributed and undistributed income (loss) available to nonvested shareholders	(600,000)	200,000
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	(482,400,000)	139,100,000
Basic shares	182.4	183.9
Basic EPS (in dollars per share)	$ (2.65)	$ 0.76	$ 0.00
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	(482,400,000)	139,100,000
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	$ (482,400,000)	$ 139,100,000
Basic shares	182.4	183.9
Effect of dilutive securities:
Options and restricted stock awards (in shares)		0.8
Equity units (in shares)		2.4
Diluted shares	182.4	187.1
Diluted EPS (in dollars per share)	$ (2.65)	$ 0.74	$ 0.00
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	18.7	4.9