Shareholders' Equity - Progression of the components of accumulated other comprehensive income (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017		Jan. 01, 2018		Dec. 31, 2017
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance						$ 813
Other comprehensive income (loss), net of tax		$ (158)		$ 116		(445)		$ 240
Cumulative effect of accounting change												$ 4
AOCI ending balance		147				147
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		(187)		178		(540)		369
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, tax		39		(63)		113		(129)
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, net of tax		(148)		115		(427)		240
Reclassification from accumulated other comprehensive income, pretax	[1]	(4)		(8)		(2)		(8)
Reclassification from accumulated other comprehensive income, tax	[1]	1		3		1		3
Reclassification from accumulated other comprehensive income, net of tax		(3)		(5)		(1)		(5)
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		(191)		170		(542)		361
Other comprehensive income (loss), tax		40		(60)		114		(126)
Other comprehensive income (loss), net of tax		(151)		110		(428)		235
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		0
Reclassification from accumulated other comprehensive income, net of tax	[1]	0		0		0		0
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax		0		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		(148)		115		(427)		240
Reclassification from accumulated other comprehensive income, net of tax		(3)		(5)		(1)		(5)
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		342	[2]	529		840	[2]	404
Other comprehensive income (loss), net of tax		(151)		110		(428)		235
Cumulative effect of accounting change	[3]	0		0		0		0		$ (221)
AOCI ending balance		191	[2]	639		191	[2]	639
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		(4)		4		(18)		2
Other comprehensive income (loss), tax		1		(2)		4		(1)
Other comprehensive income (loss), net of tax		(3)		2		(14)		1
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		0
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(24)		(8)		(13)		(7)
Other comprehensive income (loss), net of tax		(3)		2		(14)		1
Cumulative effect of accounting change	[3]	0		0		0		0		0
AOCI ending balance		(27)		(6)		(27)		(6)
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		(4)		3		(2)		3
Other comprehensive income (loss), tax		0		1		(1)		1
Other comprehensive income (loss), net of tax		(4)		4		(3)		4
Accumulated Foreign Currency Adjustment Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax		0		0		0		0
Accumulated Foreign Currency Adjustment Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(5)		(15)		(6)		(15)
Other comprehensive income (loss), net of tax		(4)		4		(3)		4
Cumulative effect of accounting change	[3]	0		0		0		0		0
AOCI ending balance		(9)		(11)		(9)		(11)
Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		0		0		0		0
Other comprehensive income (loss), tax		0		0		0		0
Other comprehensive income (loss), net of tax		0		0		0		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		0
Accumulated Defined Benefit Plans Adjustment Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(8)		(7)		(8)		(7)
Other comprehensive income (loss), net of tax		0		0		0		0
Cumulative effect of accounting change	[3]	0		0		0		0		0
AOCI ending balance		(8)		(7)		(8)		(7)
AOCI Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		(199)		177		(562)		366
Other comprehensive income (loss), tax		41		(61)		117		(126)
Other comprehensive income (loss), net of tax		(158)		116		(445)		240
AOCI Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax		0		0		0		0
AOCI Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		305		499		813		375
Other comprehensive income (loss), net of tax		(158)		116		(445)		240
Cumulative effect of accounting change		0	[3]	0	[3]	0	[3]	0	[3]	$ (221)	[3]	$ (221)
AOCI ending balance		$ 147		$ 615		$ 147		$ 615

[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 $67 million at June 30, 2018 and $68 million at both March 31, 2018 and December 31, 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.