Shareholders' Equity - Progression of the components of accumulated other comprehensive income (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2018		Sep. 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		$ (103)		$ 74		(548)		$ 314
Cumulative effect of accounting change												$ 4
AOCI ending balance		44				44
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		(122)		92		(662)		461
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, tax		26		(33)		139		(162)
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, net of tax		(96)		59		(523)		299
Reclassification from accumulated other comprehensive income, pretax	[1]	(2)		12		(4)		4
Reclassification from accumulated other comprehensive income, tax	[1]	0		(4)		1		(1)
Reclassification from accumulated other comprehensive income, net of tax		(2)		8		(3)		3
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		(124)		104		(666)		465
Other comprehensive income (loss), tax		26		(37)		140		(163)
Other comprehensive income (loss), net of tax		(98)		67		(526)		302
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		(96)		59		(523)		299
Reclassification from accumulated other comprehensive income, net of tax		(2)		8		(3)		3
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		191	[2]	639		840	[2]	404
Other comprehensive income (loss), net of tax		(98)		67		(526)		302
Cumulative effect of accounting change	[3]	0		0		0		0		$ (221)
AOCI ending balance		93	[2]	706		93	[2]	706
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		(6)		(1)		(24)		1
Other comprehensive income (loss), tax		1		1		5		0
Other comprehensive income (loss), net of tax		(5)		0		(19)		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		(27)		(6)		(13)		(7)
Other comprehensive income (loss), net of tax		(5)		0		(19)		1
Cumulative effect of accounting change	[3]	0		0		0		0		0
AOCI ending balance		(32)		(6)		(32)		(6)
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		0		5		(2)		8
Other comprehensive income (loss), tax		0		2		(1)		3
Other comprehensive income (loss), net of tax		0		7		(3)		11
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		(9)		(11)		(6)		(15)
Other comprehensive income (loss), net of tax		0		7		(3)		11
Cumulative effect of accounting change	[3]	0		0		0		0		0
AOCI ending balance		(9)		(4)		(9)		(4)
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		(130)		108		(692)		474
Other comprehensive income (loss), tax		27		(34)		144		(160)
Other comprehensive income (loss), net of tax		(103)		74		(548)		314
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		147		615		813		375
Other comprehensive income (loss), net of tax		(103)		74		(548)		314
Cumulative effect of accounting change		0	[3]	0	[3]	0	[3]	0	[3]	$ (221)	[3]	$ (221)
AOCI ending balance		$ 44		$ 689		$ 44		$ 689

[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 $64 million at September 30, 2018 compared to $67 million at June 30, 2018 and $68 million at 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.