Shareholders' Equity - Progression of the components of accumulated other comprehensive income (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2017		Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016	Jun. 30, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net of tax unrealized gains (losses) related to credit-only impaired securities		$ 58			$ 58			$ 56	$ 52
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance					375
Other comprehensive income, net of tax		74		$ 85	314		$ 456
AOCI ending balance		689			689
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		92		138	461		656
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, tax		(33)		(49)	(162)		(229)
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, net of tax		59		89	299		427
Reclassification from accumulated other comprehensive income, pretax	[1]	12		(2)	4		32
Reclassification from accumulated other comprehensive income, tax	[1]	(4)		1	(1)		(12)
Reclassification from accumulated other comprehensive income, net of tax		8		(1)	3		20
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		104		136	465		688
Other comprehensive income (loss), tax		(37)		(48)	(163)		(241)
Other comprehensive income, net of tax		67		88	302		447
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		(1)	0		(6)
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		(1)	0		(7)
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		59		88	299		421
Reclassification from accumulated other comprehensive income, net of tax		8		(1)	3		19
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		639	[2]	685	404	[2]	332
Other comprehensive income, net of tax		67		87	302		440
AOCI ending balance		706	[2]	772	706	[2]	772
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		(1)		(1)	1		6
Other comprehensive income (loss), tax		1		1	0		(2)
Other comprehensive income, net of tax		0		0	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		(6)		5	(7)		1
Other comprehensive income, net of tax		0		0	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		5		(2)	8		2
Other comprehensive income (loss), tax		2		(1)	3		2
Other comprehensive income, net of tax		7		(3)	11		4
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		(11)		(15)	(15)		(22)
Other comprehensive income, net of tax		7		(3)	11		4
AOCI ending balance		(4)		(18)	(4)		(18)
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		108		133	474		697
Other comprehensive income (loss), tax		(34)		(48)	(160)		(241)
Other comprehensive income, net of tax		74		85	314		456
AOCI Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income, net of tax		0		(1)	0		(7)
AOCI Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		615		669	375		304
Other comprehensive income, net of tax		74		84	314		449
AOCI ending balance		$ 689		$ 753	$ 689		$ 753

[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 $58 million at September 30, 2017 compared to $56 million at June 30, 2017 and $52 million at December 31, 2016 related to securities for which only the credit portion of an other-than-temporary impairment has been recorded in earnings.