Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic EPS:
Net income (loss) attributable to AGL	$ 208	$ 479	$ 678	$ 684
Less: Distributed and undistributed income (loss) available to nonvested shareholders	0	1	1	1
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 208	$ 478	$ 677	$ 683
Basic shares	118.7	131.9	121.8	134.0
Basic EPS (in dollars per share)	$ 1.75	$ 3.63	$ 5.56	$ 5.10
Diluted EPS:
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, diluted	$ 208	$ 478	$ 677	$ 683
Basic shares	118.7	131.9	121.8	134.0
Effect of dilutive securities:
Options and restricted stock awards (in shares)	2.0	0.9	1.7	0.9
Diluted shares	120.7	132.8	123.5	134.9
Diluted EPS (in dollars per share)	$ 1.72	$ 3.60	$ 5.48	$ 5.06
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.0	0.0	0.1	0.5