Accounting Policies - Narrative (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Net unrealized gain on equity securities included in AOCI		$ 342			$ 840
Cumulative effect of accounting change					4
Realized gains (losses) on securities		$ (93)	$ 3
Weighted average common shares adjustment related to stock-based compensation (shares)		1,800,000	2,100,000
Anti-dilutive potential common shares related to stock-based compensation plans (shares)		0	0
Maturities of short term investments		3 months
Equity securities
Significant Accounting Policies [Line Items]
Available-for-sale securities, Equity securities					1,600
Net unrealized gain on equity securities included in AOCI	[1]				221
Equity securities, trading at fair value					62
Realized gains (losses) on securities	[2]	$ (95)	$ (7)
Equity securities | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Net unrealized gain on equity securities included in AOCI					$ 221
Cumulative effect of accounting change				$ (221)
Realized gains (losses) on securities		$ (94)
Limited partnerships and similar investments | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change				$ (4)

[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.

[2] 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.