Shareholders' Equity - Progression of the components of accumulated other comprehensive income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017		Mar. 31, 2016
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		$ 375
Other comprehensive income, net of tax		124		$ 146
AOCI ending balance		499
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		191		192
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, tax		(66)		(67)
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, net of tax		125		125
Reclassification from accumulated other comprehensive income, pretax	[1]	0		18
Reclassification from accumulated other comprehensive income, tax	[1]	0		(7)
Reclassification from accumulated other comprehensive income, net of tax		0		11
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		191		210
Other comprehensive income (loss), tax		(66)		(74)
Other comprehensive income, net of tax		125		136
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)
Reclassification from accumulated other comprehensive income, net of tax	[1]	0		(1)
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income, net of tax		0		(2)
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		125		124
Reclassification from accumulated other comprehensive income, net of tax		0		10
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		404	[2]	332
Other comprehensive income, net of tax		125		134
AOCI ending balance		529	[2]	466
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		(2)		5
Other comprehensive income (loss), tax		1		(2)
Other comprehensive income, net of tax		(1)		3
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
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(7)		1
Other comprehensive income, net of tax		(1)		3
AOCI ending balance		(8)		4
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		0		3
Other comprehensive income (loss), tax		0		3
Other comprehensive income, net of tax		0		6
Accumulated Foreign Currency Adjustment Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income, net of tax		0		0
Accumulated Foreign Currency Adjustment Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(15)		(22)
Other comprehensive income, net of tax		0		6
AOCI ending balance		(15)		(16)
Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		0		1
Other comprehensive income (loss), tax		0		0
Other comprehensive income, net of tax		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
Accumulated Defined Benefit Plans Adjustment Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(7)		(7)
Other comprehensive income, net of tax		0		1
AOCI ending balance		(7)		(6)
AOCI Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		189		219
Other comprehensive income (loss), tax		(65)		(73)
Other comprehensive income, net of tax		124		146
AOCI Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income, net of tax		0		(2)
AOCI Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		375		304
Other comprehensive income, net of tax		124		144
AOCI ending balance		$ 499		$ 448

[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 $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.