Shareholders' Equity - Progression of the components of accumulated other comprehensive income (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2017		Jun. 30, 2016	Jun. 30, 2017		Jun. 30, 2016
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance					$ 375
Other comprehensive income, net of tax		$ 116		$ 225	240		$ 371
AOCI ending balance		615			615
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		178		326	369		518
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, tax		(63)		(113)	(129)		(180)
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, net of tax		115		213	240		338
Reclassification from accumulated other comprehensive income, pretax	[1]	(8)		16	(8)		34
Reclassification from accumulated other comprehensive income, tax	[1]	3		(6)	3		(13)
Reclassification from accumulated other comprehensive income, net of tax		(5)		10	(5)		21
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		170		342	361		552
Other comprehensive income (loss), tax		(60)		(119)	(126)		(193)
Other comprehensive income, net of tax		110		223	235		359
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		(4)	0		(5)
Reclassification from accumulated other comprehensive income, net of tax	[1]	0		0	0		(1)
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income, net of tax		0		(4)	0		(6)
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		115		209	240		333
Reclassification from accumulated other comprehensive income, net of tax		(5)		10	(5)		20
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		529	[2]	466	404	[2]	332
Other comprehensive income, net of tax		110		219	235		353
AOCI ending balance		639	[2]	685	639	[2]	685
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		2	2		7
Other comprehensive income (loss), tax		(2)		(1)	(1)		(3)
Other comprehensive income, net of tax		2		1	1		4
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income, 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		(8)		4	(7)		1
Other comprehensive income, net of tax		2		1	1		4
AOCI ending balance		(6)		5	(6)		5
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		3		1	3		4
Other comprehensive income (loss), tax		1		0	1		3
Other comprehensive income, net of tax		4		1	4		7
Accumulated Foreign Currency Adjustment Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income, net of tax		0		0	0		0
Accumulated Foreign Currency Adjustment Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(15)		(16)	(15)		(22)
Other comprehensive income, net of tax		4		1	4		7
AOCI ending balance		(11)		(15)	(11)		(15)
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		1
Other comprehensive income (loss), tax		0		0	0		0
Other comprehensive income, net of tax		0		0	0		1
Accumulated Defined Benefit Plans Adjustment Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income, net of tax		0		0	0		0
Accumulated Defined Benefit Plans Adjustment Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(7)		(6)	(7)		(7)
Other comprehensive income, net of tax		0		0	0		1
AOCI ending balance		(7)		(6)	(7)		(6)
AOCI Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		177		345	366		564
Other comprehensive income (loss), tax		(61)		(120)	(126)		(193)
Other comprehensive income, net of tax		116		225	240		371
AOCI Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income, net of tax		0		(4)	0		(6)
AOCI Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		499		448	375		304
Other comprehensive income, net of tax		116		221	240		365
AOCI ending balance		$ 615		$ 669	$ 615		$ 669

[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 (loss) attributable to noncontrolling interests

[2] Includes net unrealized gains of $56 million at June 30, 2017 compared to $52 million at both March 31, 2017 and December 31, 2016 related to securities for which only the credit portion of an other-than-temporary impairment has been recorded in earnings.