Shareholders' Equity - Progression of the components of accumulated other comprehensive income (Details) - USD ($) $ in Millions			12 Months Ended
		Dec. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Jan. 01, 2018
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance			$ 813
Other comprehensive income (loss), net of tax			(544)		$ 296		$ 67
Cumulative effect of accounting change		$ 4			4
Other comprehensive income (loss), net of tax, acquisition of noncontrolling interest							(315)
AOCI ending balance		813	48		813
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			(689)		456		124
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, tax			145		(159)		(44)
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, net of tax			(544)		297		80
Reclassification from accumulated other comprehensive income, pretax	[1]		10		(15)		(19)
Reclassification from accumulated other comprehensive income, tax	[1]		(2)		5		7
Reclassification from accumulated other comprehensive income, net of tax			8		(10)		(12)
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax			(679)		441		105
Other comprehensive income (loss), tax			143		(154)		(37)
Other comprehensive income (loss), net of tax			(536)		287		68
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		(4)
Reclassification from accumulated other comprehensive income, net of tax	[1]		0		0		(1)
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax			0		0		(5)
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			(544)		297		76
Reclassification from accumulated other comprehensive income, net of tax			8		(10)		(13)
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance			840	[2]	404	[2]	332
Other comprehensive income (loss), net of tax			(536)		287		63
Cumulative effect of accounting change	[3]							$ (221)
Other comprehensive income (loss), net of tax, acquisition of noncontrolling interest	[3]						9
AOCI ending balance	[2]	840	83		840		404
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			2		(6)		(12)
Other comprehensive income (loss), tax			0		2		4
Other comprehensive income (loss), net of tax			2		(4)		(8)
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			(13)		(7)		1
Other comprehensive income (loss), net of tax			2		(4)		(8)
Cumulative effect of accounting change	[3]							0
AOCI ending balance		(13)	(11)		(13)		(7)
Accumulated foreign currency adjustment including portion attributable to noncontrolling interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax			(9)		9		6
Other comprehensive income (loss), tax			(1)		3		1
Other comprehensive income (loss), net of tax			(10)		12		7
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			(6)		(15)		(22)
Other comprehensive income (loss), net of tax			(10)		12		7
Cumulative effect of accounting change	[3]							0
AOCI ending balance		(6)	(16)		(6)		(15)
Accumulated defined benefit plans adjustment including portion attributable to noncontrolling interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax			0		1		0
Other comprehensive income (loss), tax			0		0		0
Other comprehensive income (loss), net of tax			0		1		0
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)		(7)		(7)
Other comprehensive income (loss), net of tax			0		1		0
Cumulative effect of accounting change	[3]							0
AOCI ending balance		(8)	(8)		(8)		(7)
AOCI including portion attributable to noncontrolling interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax			(686)		445		99
Other comprehensive income (loss), tax			142		(149)		(32)
Other comprehensive income (loss), net of tax			(544)		296		67
AOCI attributable to noncontrolling interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax			0		0		(5)
AOCI attributable to parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance			813		375		304
Other comprehensive income (loss), net of tax			(544)		296		62
Cumulative effect of accounting change		(221)			(221)			$ (221)	[3]
Other comprehensive income (loss), net of tax, acquisition of noncontrolling interest	[3]						9
AOCI ending balance		813	$ 48		$ 813		$ 375
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 of 2017, 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 of 2017, 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 of 2017, 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 of 2017, Reclassification from AOCI to Retained Earnings, Tax Effect	[3]	142
Redeemable noncontrolling interest | 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 of 2017, Reclassification from AOCI to Retained Earnings, Tax Effect	[3]	$ (3)

[1] The reclassification adjustment out of net unrealized gains 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 Attributable to noncontrolling interests Net earnings attributable to noncontrolling interests

[2] Includes net unrealized gains of $58 million at December 31, 2018 compared to net unrealized gains of $68 million and $52 million at December 31, 2017 and 2016, 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”), the impact on AOCI of the December 2017 sale of redeemable noncontrolling interests in Neon and the November 2016 acquisition of the noncontrolling interest in NATL (see Note B — “Acquisitions and Sale of Businesses”).