Fair Value Measurements (Details Textual) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 Stock	Mar. 31, 2011
Fair Value Measurements (Textual) [Abstract]
Fair value of assets transferred from Level 1 to Level 2	$ 35
Number of preferred stocks transferred from Level 1 to Level 2	6
Level 3 assets as a percentage of total assets measured at fair value	4.00%
Percentage of level 3 assets that were priced using non-binding broker quotes	94.00%
Percentage of internally developed Level 3 assets to total assets	0.50%
Total realized/unrealized losses included in net income on liabilities still outstanding	(39)	(61)
AFG's internal investment professionals	20
Level 3 assets that were priced using non-binding broker quotes	$ 949
Percentage of internally developed Level 3 assets to Shareholders' equity	2.00%