Shareholders' Equity - Progression of the components of accumulated other comprehensive income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance				$ 863
Other comprehensive income, net of tax		$ 1,021	$ 366	192	$ 762
AOCI ending balance		1,053		1,053
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		1,293	450	198	937
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, tax		(272)	(94)	(42)	(197)
Other comprehensive income (loss), unrealized holding gains on securities arising during the period, net of tax		1,021	356	156	740
Reclassification from accumulated other comprehensive income, pretax	[1]	(9)	(10)	15	(14)
Reclassification from accumulated other comprehensive income, tax	[1]	2	2	(3)	3
Reclassification from accumulated other comprehensive income, net of tax	[1]	(7)	(8)	12	(11)
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		1,284	440	213	923
Other comprehensive income (loss), tax		(270)	(92)	(45)	(194)
Other comprehensive income, net of tax		1,014	348	168	729
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	0	0
Reclassification from accumulated other comprehensive income, net of tax	[1]	0	0	0	0
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income, net of tax		0	0	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		1,021	356	156	740
Reclassification from accumulated other comprehensive income, net of tax	[1]	(7)	(8)	12	(11)
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		16	464	862	83
Other comprehensive income, net of tax		1,014	348	168	729
AOCI ending balance		1,030	812	1,030	812
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	23	38	37
Other comprehensive income (loss), tax		(1)	(5)	(8)	(8)
Other comprehensive income, net of tax		3	18	30	29
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		44	0	17	(11)
Other comprehensive income, net of tax		3	18	30	29
AOCI ending balance		47	18	47	18
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		4	(1)	(6)	3
Other comprehensive income (loss), tax		0	1	0	1
Other comprehensive income, net of tax		4	0	(6)	4
Accumulated Foreign Currency Adjustment Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income, net of tax		0	(1)	(2)	(1)
Accumulated Foreign Currency Adjustment Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		(21)	(12)	(9)	(16)
Other comprehensive income, net of tax		4	(1)	(8)	3
AOCI ending balance		(17)	(13)	(17)	(13)
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	0
Other comprehensive income (loss), tax		0	0	0	0
Other comprehensive income, net of tax		0	0	0	0
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)	(8)	(7)	(8)
Other comprehensive income, net of tax		0	0	0	0
AOCI ending balance		(7)	(8)	(7)	(8)
AOCI Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income (loss), pretax		1,292	462	245	963
Other comprehensive income (loss), tax		(271)	(96)	(53)	(201)
Other comprehensive income, net of tax		1,021	366	192	762
AOCI Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income [Roll Forward]
Other comprehensive income, net of tax		0	(1)	(2)	(1)
AOCI Attributable to Parent
Accumulated Other Comprehensive Income [Roll Forward]
AOCI beginning balance		32	444	863	48
Other comprehensive income, net of tax		1,021	365	190	761
AOCI ending balance		$ 1,053	$ 809	$ 1,053	$ 809

[1] The reclassification adjustment out of net unrealized gains (losses) on securities affected the following lines in AFG’s Statement of Earnings: OCI componentAffected line in the statement of earningsPretaxRealized gains (losses) on securitiesTaxProvision (credit) for income taxes