Condensed Financial Information of Parent Company - Condensed Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Activities:
Net earnings (loss) attributable to shareholders	$ 692	$ 164	$ 177	$ (301)	$ 211	$ 147	$ 210	$ 329	$ 732	$ 897	$ 530
Adjustments:
Net cash provided by operating activities									2,183	2,456	2,083
Investing Activities:
Net cash used in investing activities									(1,564)	(3,065)	(5,350)
Financing Activities:
Additional long-term borrowings									634	315	0
Reductions of long-term debt									(150)	(150)	0
Issuances of Common Stock									22	36	33
Repurchases of Common Stock									(313)	0	(6)
Cash dividends paid on Common Stock									(334)	(444)	(394)
Net cash provided by (used in) financing activities									(123)	1,408	2,444
Cash and cash equivalents at beginning of year				2,314				1,515	2,314	1,515	2,338
Cash and cash equivalents at end of year	2,810				2,314				2,810	2,314	1,515
AFG
Operating Activities:
Net earnings (loss) attributable to shareholders									732	897	530
Adjustments:
Equity in net earnings of subsidiaries									(780)	(1,032)	(637)
Dividends from subsidiaries									543	408	238
Other operating activities, net									(12)	33	84
Net cash provided by operating activities									483	306	215
Investing Activities:
Capital contributions to subsidiaries									(297)	(60)	(11)
Returns of capital from subsidiaries									0	4	23
Purchases of investments, property and equipment									(2)	(3)	(5)
Proceeds from maturities and redemptions of investments									2	3	3
Proceeds from sales of businesses									3	0	0
Net cash used in investing activities									(294)	(56)	10
Financing Activities:
Additional long-term borrowings									634	315	0
Reductions of long-term debt									(150)	(150)	0
Issuances of Common Stock									23	37	34
Repurchases of Common Stock									(313)	0	(6)
Cash dividends paid on Common Stock									(334)	(444)	(394)
Net cash provided by (used in) financing activities									(140)	(242)	(366)
Cash and cash equivalents, period increase (decrease)									49	8	(141)
Cash and cash equivalents at beginning of year				$ 166				$ 158	166	158	299
Cash and cash equivalents at end of year	$ 215				$ 166				$ 215	$ 166	$ 158