Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic EPS:
Net income (loss)	$ 52	$ 208	$ 153	$ 317	$ 197	$ 479	$ 146	$ 59	$ 730	$ 881	$ 1,056
Less: Distributed and undistributed income (loss) available to nonvested shareholders									1	1	1
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic									$ 729	$ 880	$ 1,055
Basic shares (in shares)									120.6	133.0	148.1
Basic EPS (in dollars per share)	$ 0.44	$ 1.75	$ 1.26	$ 2.53	$ 1.51	$ 3.63	$ 1.09	$ 0.43	$ 6.05	$ 6.61	$ 7.12
Diluted EPS:
Plus: Re-allocation of undistributed income (loss) available to nonvested shareholders of AGL and subsidiaries									$ 0	$ 0	$ 0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, diluted									$ 729	$ 880	$ 1,055
Basic shares (in shares)									120.6	133.0	148.1
Dilutive securities (in shares)									1.7	1.1	0.9
Diluted shares (in shares)									122.3	134.1	149.0
Diluted EPS (in dollars per share)	$ 0.44	$ 1.72	$ 1.24	$ 2.49	$ 1.49	$ 3.60	$ 1.09	$ 0.43	$ 5.96	$ 6.56	$ 7.08
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect									0.1	0.3	0.5