Earnings Per Share (Details) (USD $) In Millions, except Per Share data	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Basic earnings per share:
Net income (loss)	$ (57.7)	$ 203.5	$ 67.7	$ 525.5	$ 548.9	$ 97.2	$ 68.9
Less: Distributed and undistributed income (loss) available to nonvested shareholders		0.2	0.1	0.7
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	(57.7)	203.3	67.6	524.8
Basic shares (in shares)	184.2	184.1	184.0	184.2
Basic earnings per share (in dollars per share)	$ (0.31)	$ 1.10	$ 0.37	$ 2.85
Diluted earnings per share:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	(57.7)	203.3	67.6	524.8
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries	$ (57.7)	$ 203.3	$ 67.6	$ 524.8
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)	$ (0.31)	$ 1.08	$ 0.36	$ 2.77
Potentially dilutive securities excluded from computation of earnings per share because of antidilutive effect (in shares)	9.6	2.7	2.9	2.4