Earnings Per Share (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Basic EPS:
Net income (loss) attributable to AGL	$ (83,600,000)	$ 761,200,000	$ (42,600,000)	$ 140,600,000	$ (183,500,000)	$ 164,600,000	$ 179,100,000	$ 333,500,000	$ 775,620,000	$ 493,707,000	$ 85,978,000
Less: Distributed and undistributed income (loss) available to nonvested shareholders									500,000	900,000	300,000
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries									775,100,000	492,800,000	85,700,000
Basic shares									183.4	184.0	126.5
Basic EPS (in dollars per share)	$ (0.46)	$ 4.15	$ (0.23)	$ 0.76	$ (1.00)	$ 0.89	$ 0.97	$ 1.81	$ 4.23	$ 2.68	$ 0.68
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries									775,100,000	492,800,000	85,700,000
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries									$ 775,100,000	$ 492,800,000	$ 85,700,000
Basic shares									183.4	184.0	126.5
Effect of dilutive securities:
Options and restricted stock awards (in shares)									0.9	0.9	0.6
Equity units (in shares)									1.2	4.0	2.0
Diluted shares									185.5	188.9	129.1
Diluted EPS (in dollars per share)	$ (0.46)	$ 4.13	$ (0.23)	$ 0.75	$ (1.00)	$ 0.88	$ 0.95	$ 1.75	$ 4.18	$ 2.61	$ 0.66
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)									7.2	3.0	4.4