Shareholders' Equity - Progression of the components of accumulated other comprehensive income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019		Mar. 31, 2018		Jan. 01, 2018		Dec. 31, 2017
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		$ 48
Other comprehensive income (loss), net of tax		396		$ (287)
Cumulative effect of accounting change								$ 4
AOCI ending balance		444
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		487		(353)
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, tax		(103)		74
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, net of tax		384		(279)
Reclassification from accumulated other comprehensive income, pretax	[1]	(4)		2
Reclassification from accumulated other comprehensive income, tax	[1]	1		0
Reclassification from accumulated other comprehensive income, net of tax		(3)		2
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		483		(351)
Other comprehensive income (loss), tax		(102)		74
Other comprehensive income (loss), net of tax		381		(277)
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
Reclassification from accumulated other comprehensive income, net of tax	[1]	0		0
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax		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		384		(279)
Reclassification from accumulated other comprehensive income, net of tax		(3)		2
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		83	[2]	840
Other comprehensive income (loss), net of tax		381		(277)
Cumulative effect of accounting change	[3]			(221)		$ 0
AOCI ending balance		464	[2]	342
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		14		(14)
Other comprehensive income (loss), tax		(3)		3
Other comprehensive income (loss), net of tax		11		(11)
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
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(11)		(13)
Other comprehensive income (loss), net of tax		11		(11)
Cumulative effect of accounting change	[3]			0		0
AOCI ending balance		0		(24)
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		4		2
Other comprehensive income (loss), tax		0		(1)
Other comprehensive income (loss), net of tax		4		1
Accumulated Foreign Currency Adjustment Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax		0		0
Accumulated Foreign Currency Adjustment Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(16)		(6)
Other comprehensive income (loss), net of tax		4		1
Cumulative effect of accounting change	[3]			0		0
AOCI ending balance		(12)		(5)
Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		0		0
Other comprehensive income (loss), tax		0		0
Other comprehensive income (loss), net of tax		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
Accumulated Defined Benefit Plans Adjustment Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(8)		(8)
Other comprehensive income (loss), net of tax		0		0
Cumulative effect of accounting change	[3]			0		0
AOCI ending balance		(8)		(8)
AOCI Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		501		(363)
Other comprehensive income (loss), tax		(105)		76
Other comprehensive income (loss), net of tax		396		(287)
AOCI Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax		0		0
AOCI Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		48		813
Other comprehensive income (loss), net of tax		396		(287)
Cumulative effect of accounting change				(221)	[3]	$ 0	[3]	$ (221)
AOCI ending balance		$ 444		$ 305

[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 $61 million at March 31, 2019 compared to $58 million at December 31, 2018 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.