Shareholders' Equity - Deferred Compensation (Details)	12 Months Ended
	Dec. 31, 2014 Unit	Dec. 31, 2013 Unit
Defined Benefit Plan Disclosure [Line Items]
Deferred equity compensation, shares	320,193	320,193
AGC [Member] | Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of AGL common shares represented by each unit in employer stock fund	1
Number of units in AGC SERP (in units)	74,309	74,309
Chief Executive Officer and General Counsel [Member] | Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of AGL common shares represented by each unit in employer stock fund	1
Deferred equity compensation, shares	320,193	320,193