Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Basic EPS:
Net income (loss)	$ 201us-gaap_NetIncomeLoss	$ 42us-gaap_NetIncomeLoss
Less: Distributed and undistributed income (loss) available to nonvested shareholders	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, basic	201	42
Basic shares	155.8	182.1
Basic EPS (in dollars per share)	$ 1.29us-gaap_EarningsPerShareBasic	$ 0.23us-gaap_EarningsPerShareBasic
Diluted EPS:
Plus: Re-allocation of undistributed income (loss) available to nonvested shareholders of AGL and subsidiaries	0	0
Distributed and undistributed income (loss) available to common shareholders of AGL and subsidiaries, diluted	$ 201	$ 42
Basic shares	155.8	182.1
Effect of dilutive securities:
Options and restricted stock awards (in shares)	1.0	1.0
Diluted shares	156.8	183.1
Diluted EPS (in dollars per share)	$ 1.28us-gaap_EarningsPerShareDiluted	$ 0.23us-gaap_EarningsPerShareDiluted
Potentially dilutive securities excluded from computation of EPS because of antidilutive effect	0.7	1.5

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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