Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic EPS:
Net income (loss) attributable to AGL	$ 137	$ 69	$ 142	$ 54	$ 88	$ 161	$ 75	$ 197	$ 402	$ 521	$ 730
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									$ 401	$ 520	$ 729
Basic shares (in shares)									99.3	110.0	120.6
Basic EPS (in dollars per share)	$ 1.43	$ 0.71	$ 1.40	$ 0.52	$ 0.84	$ 1.48	$ 0.67	$ 1.71	$ 4.04	$ 4.73	$ 6.05
Diluted EPS:
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic									$ 401	$ 520	$ 729
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									$ 401	$ 520	$ 729
Basic shares (in shares)									99.3	110.0	120.6
Dilutive securities: Options and restricted stock awards (in shares)									0.9	1.3	1.7
Diluted shares (in shares)									100.2	111.3	122.3
Diluted EPS (in dollars per share)	$ 1.42	$ 0.70	$ 1.39	$ 0.52	$ 0.83	$ 1.47	$ 0.67	$ 1.68	$ 4.00	$ 4.68	$ 5.96
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect									0.0	0.1	0.1