Shareholders' Equity (Details) (USD $)	0 Months Ended				3 Months Ended																12 Months Ended									10 Months Ended	12 Months Ended	0 Months Ended		12 Months Ended		12 Months Ended
	Feb. 07, 2013	Jun. 01, 2012		Feb. 09, 2012	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013 vote		Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2013 Common Stock [Member]	Dec. 31, 2012 Common Stock [Member]	Dec. 31, 2011 Common Stock [Member]	Nov. 11, 2013 Common Stock [Member]	Dec. 31, 2013 Common Stock [Member]	Jun. 05, 2013 Common Stock [Member] Limited Liability Company [Member]	Feb. 05, 2014 Subsequent Event [Member]	Dec. 31, 2013 Supplemental Employee Retirement Plan, Defined Benefit [Member]	Dec. 31, 2012 Supplemental Employee Retirement Plan, Defined Benefit [Member]	Dec. 31, 2013 Supplemental Employee Retirement Plan, Defined Benefit [Member] AGC Unit	Dec. 31, 2012 Supplemental Employee Retirement Plan, Defined Benefit [Member] AGC Unit
Share Issuances
Authorized share capital					$ 5,000,000																$ 5,000,000
Common stock, shares authorized					500,000,000								500,000,000								500,000,000		500,000,000
Common stock par value (in dollars per share)					$ 0.01								$ 0.01								$ 0.01		$ 0.01
Number of votes per share of common stock																					1
Percentage of controlled shares																					9.50%
Percentage of voting power held by one shareholder																					75.00%
Issuance of Shares, Number of Shares		13,428,770	[1]																									13,428,770
Issuance of Shares, Price per Share (in dollars per share)		$ 12.85	[1]
Issuance of Shares, Proceeds		173,000,000	[1]
Issuance of Shares, Net Proceeds		173,000,000	[1]																		0		173,000,000		0
Share Repurchases
Share repurchase program, authorized amount																															400,000,000
Share repurchase																					264,000,000				23,000,000		0	0	0	264,000,000		109,700,000
Share repurchase, average price per share																														$ 21.12
Number of shares held by related party																																14,900,000
Percentage of ownership by related party																																8.00%
Previous percentage of ownership by related party																																10.50%
Number of Shares Repurchased																											12,512,759	2,066,759	2,000,000	12,500,000		5,000,000
Total Payments																					$ 264,000,000		$ 24,000,000		$ 23,000,000
Deferred Compensation
Number of AGL common shares represented by each unit in employer stock fund																																		1		1
Number of shares in AGL SERP (in shares)					320,193								320,193								320,193		320,193											320,193	320,193
Number of units in AGC SERP (in units)																																				74,309	68,181
Dividends
Dividends, per share (in dollars per share)	$ 0.10			$ 0.09	$ 0.10	[2]	$ 0.10	[2]	$ 0.10	[2]	$ 0.10	[2]	$ 0.09	[2]	$ 0.09	[2]	$ 0.09	[2]	$ 0.09	[2]	$ 0.40	[2]	$ 0.36	[2]	$ 0.18								$ 0.11
Increase in common stock dividend (as a percent)	11.00%			100.00%																													10.00%
Dividend paid on common share (in dollars per share)																									$ 0.045	$ 0.045

[1] Relates to the settlement of forward purchase contracts. See Note 17, Long-Term Debt and Credit Facilities.

[2] Per share amounts for the quarters and the full years have each been calculated separately. Accordingly, quarterly amounts may not sum up to the annual amounts because of differences in the average common shares outstanding during each period and, with regard to diluted per share amounts only, because of the inclusion of the effect of potentially dilutive securities only in the periods in which such effect would have been dilutive.