Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic EPS:
Net income (loss) attributable to AGL	$ 75	$ 153	$ 272	$ 470
Less: Distributed and undistributed income (loss) available to nonvested shareholders	0	0	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 75	$ 153	$ 272	$ 470
Basic shares	111.7	121.3	113.4	123.3
Basic EPS (in dollars per share)	$ 0.67	$ 1.26	$ 2.39	$ 3.81
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	$ 75	$ 153	$ 272	$ 470
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	$ 75	$ 153	$ 272	$ 470
Basic shares	111.7	121.3	113.4	123.3
Effect of dilutive securities:
Options and restricted stock awards (in shares)	1.2	1.4	1.4	1.6
Diluted shares	112.9	122.7	114.8	124.9
Diluted EPS (in dollars per share)	$ 0.67	$ 1.24	$ 2.37	$ 3.76
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.1	0.1	0.2	0.1