Condensed Financial Information of Parent Company - Condensed Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities:
Net earnings attributable to shareholders	$ 211	$ 147	$ 210	$ 329	$ (29)	$ 204	$ 210	$ 145	$ 897	$ 530	$ 475
Adjustments:
Net cash provided by operating activities									2,456	2,083	1,804
Investing Activities:
Payments for (proceeds from) other investing activities									(3)	30	2
Net cash used in investing activities									(3,065)	(5,350)	(3,292)
Financing Activities:
Additional long-term borrowings									315	0	712
Reductions of long-term debt									(150)	0	(745)
Issuances of Common Stock									36	33	37
Repurchases of Common Stock									0	(6)	0
Cash dividends paid on Common Stock									(444)	(394)	(417)
Proceeds from (payments for) other financing activities									0	0	(4)
Net cash provided by (used in) financing activities									1,408	2,444	1,719
Net Change in Cash and Cash Equivalents									799	(823)	231
Cash and cash equivalents at beginning of year				1,515				2,338	1,515	2,338	2,107
Cash and cash equivalents at end of year	2,314				1,515				2,314	1,515	2,338
AFG
Operating Activities:
Net earnings attributable to shareholders									897	530	475
Adjustments:
Equity in net earnings of subsidiaries									(1,032)	(637)	(575)
Dividends from subsidiaries									408	238	580
Other operating activities, net									33	84	98
Net cash provided by operating activities									306	215	578
Investing Activities:
Capital contributions to subsidiaries									(60)	(11)	(93)
Returns of capital from subsidiaries									4	23	30
Purchases of investments, property and equipment									(3)	(5)	(2)
Proceeds from maturities and redemptions of investments									3	3	2
Net cash used in investing activities									(56)	10	(63)
Financing Activities:
Additional long-term borrowings									315	0	712
Reductions of long-term debt									(150)	0	(745)
Issuances of Common Stock									37	34	37
Repurchases of Common Stock									0	(6)	0
Cash dividends paid on Common Stock									(444)	(394)	(417)
Net cash provided by (used in) financing activities									(242)	(366)	(413)
Net Change in Cash and Cash Equivalents									8	(141)	102
Cash and cash equivalents at beginning of year				$ 158				$ 299	158	299	197
Cash and cash equivalents at end of year	$ 166				$ 158				$ 166	$ 158	$ 299