Shareholders' Equity - Deferred Compensation (Details) - AGC - Supplemental Employee Retirement Plan	12 Months Ended
	Dec. 31, 2020 Unit shares	Dec. 31, 2019 Unit
Defined Benefit Plan Disclosure [Line Items]
Number of AGL common shares represented by each unit in employer stock fund (in shares) | shares	1
Number of units in AGC SERP (in units) | Unit	74,309	74,309