Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic EPS:
Net income (loss) attributable to AGL	$ 161	$ 208	$ 433	$ 678
Less: Distributed and undistributed income (loss) available to nonvested shareholders	1	0	1	1
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 160	$ 208	$ 432	$ 677
Basic shares	108.0	118.7	111.6	121.8
Basic EPS (in dollars per share)	$ 1.48	$ 1.75	$ 3.87	$ 5.56
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 160	$ 208	$ 432	$ 677
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	$ 160	$ 208	$ 432	$ 677
Basic shares	108.0	118.7	111.6	121.8
Effect of dilutive securities:
Options and restricted stock awards (in shares)	1.3	2.0	1.3	1.7
Diluted shares	109.3	120.7	112.9	123.5
Diluted EPS (in dollars per share)	$ 1.47	$ 1.72	$ 3.83	$ 5.48
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.0	0.0	0.1	0.1