Shareholders' Equity - Progression of the components of accumulated other comprehensive income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		$ 863
Other comprehensive income (loss), net of tax		(829)	$ 396
AOCI ending balance		32
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		(1,095)	487
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, tax		230	(103)
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, net of tax		(865)	384
Reclassification from accumulated other comprehensive income, pretax	[1]	24	(4)
Reclassification from accumulated other comprehensive income, tax	[1]	(5)	1
Reclassification from accumulated other comprehensive income, net of tax		19	(3)
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		(1,071)	483
Other comprehensive income (loss), tax		225	(102)
Other comprehensive income (loss), net of tax		(846)	381
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		(865)	384
Reclassification from accumulated other comprehensive income, net of tax		19	(3)
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		862	83
Other comprehensive income (loss), net of tax		(846)	381
AOCI ending balance		16	464
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		34	14
Other comprehensive income (loss), tax		(7)	(3)
Other comprehensive income (loss), net of tax		27	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		17	(11)
Other comprehensive income (loss), net of tax		27	11
AOCI ending balance		44	0
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		(10)	4
Other comprehensive income (loss), tax		0	0
Other comprehensive income (loss), net of tax		(10)	4
Accumulated Foreign Currency Adjustment Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax		(2)	0
Accumulated Foreign Currency Adjustment Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(9)	(16)
Other comprehensive income (loss), net of tax		(12)	4
AOCI ending balance		(21)	(12)
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		(7)	(8)
Other comprehensive income (loss), net of tax		0	0
AOCI ending balance		(7)	(8)
AOCI Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		(1,047)	501
Other comprehensive income (loss), tax		218	(105)
Other comprehensive income (loss), net of tax		(829)	396
AOCI Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), net of tax		(2)	0
AOCI Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		863	48
Other comprehensive income (loss), net of tax		(831)	396
AOCI ending balance		$ 32	$ 444

[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 (credit) for income taxes