Shareholders' Equity - Progression of the components of accumulated other comprehensive income (Details) - USD ($) $ in Millions			12 Months Ended
		Dec. 31, 2017	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017	Jan. 01, 2018
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance			$ 48
Other comprehensive income (loss), net of tax			815		$ (544)		$ 296
Cumulative effect of accounting change								$ 4
Tax Cuts and Jobs Act, reclassification from AOCI to retained earnings, tax effect							0
AOCI ending balance			863		48
Accumulated net investment gain (loss) including portion attributable to noncontrolling interest
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, Pretax			997		(689)		456
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, tax			(209)		145		(159)
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, net of tax			788		(544)		297
Reclassification from accumulated other comprehensive income, pretax	[1]		(11)		10		(15)
Reclassification from accumulated other comprehensive income, tax	[1]		2		(2)		5
Reclassification from accumulated other comprehensive income, net of tax			(9)		8		(10)
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax			986		(679)		441
Other comprehensive income (loss), tax			(207)		143		(154)
Other comprehensive income (loss), net of tax			779		(536)		287
Accumulated net investment gain (loss) attributable to noncontrolling interest
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, net of tax			0		0		0
Reclassification from accumulated other comprehensive income, net of tax	[1]		0		0		0
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax			0		0		0
Accumulated net investment gain (loss) attributable to parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, net of tax			788		(544)		297
Reclassification from accumulated other comprehensive income, net of tax			(9)		8		(10)
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance			83	[2]	840	[2]	404
Other comprehensive income (loss), net of tax			779		(536)		287
Cumulative effect of accounting change			0					(221)	[3]
AOCI ending balance	[2]	$ 840	862		83		840
Accumulated net gain (loss) from cash flow hedges including portion attributable to noncontrolling interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax			36		2		(6)
Other comprehensive income (loss), tax			(8)		0		2
Other comprehensive income (loss), net of tax			28		2		(4)
Accumulated net gain (loss) from cash flow hedges attributable to noncontrolling interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax			0		0		0
Accumulated net gain (loss) from cash flow hedges attributable to parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance			(11)		(13)		(7)
Other comprehensive income (loss), net of tax			28		2		(4)
Cumulative effect of accounting change			0					0	[3]
AOCI ending balance		(13)	17		(11)		(13)
Accumulated foreign currency adjustment including portion attributable to noncontrolling interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax			7		(9)		9
Other comprehensive income (loss), tax			0		(1)		3
Other comprehensive income (loss), net of tax			7		(10)		12
Accumulated foreign currency adjustment attributable to noncontrolling interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax			0		0		0
Accumulated foreign currency adjustment attributable to parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance			(16)		(6)		(15)
Other comprehensive income (loss), net of tax			7		(10)		12
Cumulative effect of accounting change			0					0	[3]
AOCI ending balance		(6)	(9)		(16)		(6)
Accumulated defined benefit plans adjustment including portion attributable to noncontrolling interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax			1		0		1
Other comprehensive income (loss), tax			0		0		0
Other comprehensive income (loss), net of tax			1		0		1
Accumulated defined benefit plans adjustment attributable to noncontrolling interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax			0		0		0
Accumulated defined benefit plans adjustment attributable to parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance			(8)		(8)		(7)
Other comprehensive income (loss), net of tax			1		0		1
Cumulative effect of accounting change			0					0	[3]
AOCI ending balance		(8)	(7)		(8)		(8)
AOCI including portion attributable to noncontrolling interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax			1,030		(686)		445
Other comprehensive income (loss), tax			(215)		142		(149)
Other comprehensive income (loss), net of tax			815		(544)		296
AOCI attributable to noncontrolling interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax			0		0		0
AOCI attributable to parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance			48		813		375
Other comprehensive income (loss), net of tax			815		(544)		296
Cumulative effect of accounting change			0					$ (221)	[3]
Tax Cuts and Jobs Act, reclassification from AOCI to retained earnings, tax effect							145
AOCI ending balance		813	$ 863		$ 48		$ 813
Tax Cuts and Jobs Act of 2017 | Accumulated net investment gain (loss) attributable to parent
Accumulated Other Comprehensive Income [Roll Forward]
Tax Cuts and Jobs Act, reclassification from AOCI to retained earnings, tax effect	[3]	149
Tax Cuts and Jobs Act of 2017 | Accumulated net gain (loss) from cash flow hedges attributable to parent
Accumulated Other Comprehensive Income [Roll Forward]
Tax Cuts and Jobs Act, reclassification from AOCI to retained earnings, tax effect	[3]	(2)
Tax Cuts and Jobs Act of 2017 | Accumulated defined benefit plans adjustment attributable to parent
Accumulated Other Comprehensive Income [Roll Forward]
Tax Cuts and Jobs Act, reclassification from AOCI to retained earnings, tax effect	[3]	(2)
Tax Cuts and Jobs Act of 2017 | AOCI attributable to parent
Accumulated Other Comprehensive Income [Roll Forward]
Tax Cuts and Jobs Act, reclassification from AOCI to retained earnings, tax effect	[3]	142
Redeemable noncontrolling interests | Tax Cuts and Jobs Act of 2017 | Accumulated foreign currency adjustment attributable to parent
Accumulated Other Comprehensive Income [Roll Forward]
Tax Cuts and Jobs Act, reclassification from AOCI to retained earnings, tax effect	[3]	$ (3)

[1] The reclassification adjustment out of net unrealized gains (losses) on securities affected the following lines in AFG’s Statement of Earnings:   OCI component   Affected line in the statement of earnings     Pretax   Realized gains (losses) on securities     Tax   Provision for income taxes  

[2] Includes net unrealized gains of $55 million at December 31, 2019 compared to $58 million and $68 million at December 31, 2018 and 2017, related to securities for which only the credit portion of an other-than-temporary impairment has been recorded in earnings.

[3] On January 1, 2018, AFG adopted new guidance that requires all equity securities other than those accounted for under the equity method to be reported at fair value with holding gains and losses recognized in net earnings. At the date of adoption, the $221 million net unrealized gain on equity securities classified as available for sale (with unrealized holding gains and losses reported in AOCI) under the prior guidance was reclassified from AOCI to retained earnings as the cumulative effect of an accounting change. Other also includes the December 2017 reclassification of $145 million stranded in AOCI from accounting for the Tax Cuts and Jobs Act of 2017 to retained earnings (see Note A — “Accounting Policies — Income Taxes”) and the impact on AOCI of the December 2017 sale of redeemable noncontrolling interests in Neon.