Shareholders' Equity - Deferred Compensation (Details) - Supplemental Employee Retirement Plan [Member]		12 Months Ended
	Jan. 06, 2017 shares	Dec. 31, 2017 Unit shares	Dec. 31, 2016 Unit shares
AGC [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) | Unit		74,309	74,309
Chief Executive Officer and General Counsel [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of AGL common shares represented by each unit in employer stock fund		1
Deferred Compensation Arrangement with Individual, Shares Issued	320,193
Deferred equity compensation, shares	0		320,193