Earnings Per Share (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Basic earnings per share ("EPS"):
Net income (loss)	$ 376,548,000	$ (43,105,000)	$ (106,476,000)	$ 96,151,000
Less: Distributed and undistributed income (loss) available to nonvested shareholders	500,000	0	(200,000)	100,000
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	376,000,000	(43,100,000)	(106,300,000)	96,100,000
Basic shares	186.3	184.2	184.4	184.0
Basic EPS (in dollars per share)	$ 2.02	$ (0.23)	$ (0.58)	$ 0.52
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	376,000,000	(43,100,000)	(106,300,000)	96,100,000
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	$ 376,000,000	$ (43,100,000)	$ (106,300,000)	$ 96,100,000
Basic shares	186.3	184.2	184.4	184.0
Effect of dilutive securities:
Options and restricted stock awards (in shares)	0.7	0	0	0.9
Equity units (in shares)	0	0	0	2.5
Diluted shares	187.0	184.2	184.4	187.4
Diluted EPS (in dollars per share)	$ 2.01	$ (0.23)	$ (0.58)	$ 0.51
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	13.0	19.5	15.9	2.9