Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Basic EPS:
Net income (loss) attributable to AGL	$ 153	$ 146	$ 470	$ 205
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	$ 153	$ 146	$ 470	$ 205
Basic shares	121.3	134.0	123.3	135.1
Basic EPS (in dollars per share)	$ 1.26	$ 1.09	$ 3.81	$ 1.52
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	$ 153	$ 146	$ 470	$ 205
Basic shares	121.3	134.0	123.3	135.1
Effect of dilutive securities:
Options and restricted stock awards (in shares)	1.4	0.8	1.6	0.8
Diluted shares	122.7	134.8	124.9	135.9
Diluted EPS (in dollars per share)	$ 1.24	$ 1.09	$ 3.76	$ 1.51
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect (in shares)	0.1	0.6	0.1	0.7