Consolidated Statement of Changes in Equity (Unaudited) - USD ($) $ in Millions		Total	Total	Common Shares	Common Stock and Capital Surplus	Retained Earnings	Accumulated Other Comp. Income (Loss)	Noncontrolling interests
Beginning Balance, shares at Dec. 31, 2016				86,924,399
Beginning Balance at Dec. 31, 2016		$ 4,919	$ 4,916		$ 1,198	$ 3,343	$ 375	$ 3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings (losses) attributable to parent		153	153			153
Net earnings (losses) including portion attributable to nonredeemable noncontrolling interest		155						2
Other comprehensive income (loss)		124	124				124	0
Dividends on Common Stock		(27)	(27)			(27)
Shares issued:
Exercise of stock options, shares				411,167
Exercise of stock options		15	15		15
Restricted stock awards, shares				232,250
Restricted stock awards		0	0		0
Other benefit plans, shares				54,453
Other benefit plans		5	5		5
Dividend reinvestment plan, shares				3,277
Stock-based compensation expense		8	8		8
Shares exchanged — benefit plans, shares				(32,176)
Shares exchanged — benefit plans		(3)	(3)		0	(3)
Forfeitures of restricted stock, shares				(1,699)
Forfeitures of restricted stock		0	0		0
Other		(1)						(1)
Ending Balance, shares at Mar. 31, 2017				87,591,671
Ending Balance at Mar. 31, 2017		5,195	5,191		1,226	3,466	499	4
Beginning Balance, Redeemable Noncontrolling Interests at Dec. 31, 2016		0
Ending Balance, Redeemable Noncontrolling Interests at Mar. 31, 2017		0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change	[1]						0
Cumulative effect of accounting change		$ 4	4			225	(221)
Beginning Balance, shares at Dec. 31, 2017		88,275,460		88,275,460
Beginning Balance at Dec. 31, 2017		$ 5,331	5,330		1,269	3,248	813	1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings (losses) attributable to parent		145	145			145
Net earnings (losses) including portion attributable to nonredeemable noncontrolling interest		144						(1)
Other comprehensive income (loss)		(287)	(287)				(287)	0
Dividends on Common Stock		(31)	(31)			(31)
Shares issued:
Exercise of stock options, shares				374,314
Exercise of stock options		14	14		14
Restricted stock awards, shares				200,625
Restricted stock awards		0	0		0
Other benefit plans, shares				52,583
Other benefit plans		6	6		6
Dividend reinvestment plan, shares				2,779
Dividend reinvestment plan		0	0		0
Stock-based compensation expense		5	5		5
Shares exchanged — benefit plans, shares				(23,882)
Shares exchanged — benefit plans		(3)	(3)		0	(3)
Forfeitures of restricted stock, shares				(666)
Forfeitures of restricted stock		0	0		0
Other		$ 0						0
Ending Balance, shares at Mar. 31, 2018		88,881,213		88,881,213
Ending Balance at Mar. 31, 2018		$ 5,183	$ 5,183		$ 1,294	$ 3,584	$ 305	$ 0
Beginning Balance, Redeemable Noncontrolling Interests at Dec. 31, 2017		3
Redeemable Noncontrolling Interest, Equity, Carrying Amount [Abstract]
Net earnings attributable to redeemable noncontrolling interests		(3)
Ending Balance, Redeemable Noncontrolling Interests at Mar. 31, 2018		$ 0

[1] On January 1, 2018, AFG adopted new guidance that requires all equity securities other than those accounted for under the equity method to be reported at fair value with holding gains and losses recognized in net earnings. At the date of adoption, the $221 million net unrealized gain on equity securities classified as available for sale (with unrealized holding gains and losses reported in AOCI) under the prior guidance was reclassified from AOCI to retained earnings as the cumulative effect of an accounting change.