Investments - Narrative (Details) $ in Millions		3 Months Ended
		Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Investment [Line Items]
Percentage (based on amount of unrealized loss) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade		90.00%
Percentage (based on fair value) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade		94.00%
Other than temporary impairment charges		$ 1	$ 6
Residential MBS
Investment [Line Items]
Non-credit related portion of other-than-temporary impairment charges taken for securities still owned		155		$ 158
Gross unrealized losses on fixed maturities		(7)		(6)
Other than temporary impairment charges		1
Securities with non-credit other-than-temporary impairment charges
Investment [Line Items]
Gross unrealized gains		136		137
Gross unrealized losses		(4)		(4)
Fixed maturities
Investment [Line Items]
Gross unrealized losses on fixed maturities		$ (263)		$ (82)
Number of available for sale securities in an unrealized loss position | security		1,621
Other than temporary impairment charges		$ 1	0
Gross gains		6	5
Gross losses		(3)	0
Corporate bonds
Investment [Line Items]
Other than temporary impairment charges		1
Equity securities
Investment [Line Items]
Other than temporary impairment charges	[1]	$ 0	9
Gross gains			4
Gross losses			$ (2)

[1] As discussed in Note A — “Accounting Policies — Investments,” effective January 1, 2018, all equity securities other than those accounted for under the equity method are carried at fair value through net earnings. AFG recorded a net realized loss of $94 million on equity securities during the first three months of 2018 on securities that were still owned at March 31, 2018.