Acquisitions and Sale of Businesses (Details Textual) $ in Millions				3 Months Ended			9 Months Ended	12 Months Ended
	Dec. 24, 2015 USD ($) shares	Apr. 01, 2014 USD ($)	Mar. 27, 2014 USD ($)	Dec. 31, 2015 USD ($) policy	Sep. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) policy	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Nov. 30, 2015	Dec. 31, 2012 USD ($)
Acquisitions and Sale of Businesses [Line Items]
Payments to acquire business								$ 0	$ 267	$ 0
Weighted average useful life of finite-lived intangible assets acquired								5 years
Goodwill				$ 199			$ 201	$ 199	201	185		$ 185
Cash proceeds from the sale of business								7	0	0
Long-term care insurance reserves				34				34
Realized gains (losses) on subsidiaries								(161)	0	(4)
Run-off long-term care insurance business [Member]
Acquisitions and Sale of Businesses [Line Items]
Cash proceeds from the sale of business	$ 7
Investment securities proceeds received from the sale of business	11
Potential additional sale proceeds based on the release of certain statutory liabilities of the legal entities sold								13
Contingent capital support agreement amount	$ 35
Contingent capital support agreement term	5 years
Contingent capital support agreement consideration received | shares	2,000,000
Long-term care insurance reserves of business sold as a percentage of total long-term care insurance reserves											96.00%
Cash and investment securities proceeds from the sale of business	$ 18
Expected tax benefits proceeds from the sale of business								97
Loss on subsidiaries, pretax	(166)			$ (4)		$ (162)		(166)
Loss on subsidiaries, net of tax	$ (108)					$ (105)
Carrying value of the assets and liabilities of business sold as a percentage of total assets and liabilities	4.00%
Summit Holding Southeast, Inc [Member]
Acquisitions and Sale of Businesses [Line Items]
Payments to acquire business		$ 259
Capital contributions to acquired business		140
Total capital investment in acquired business		399
Net earned premiums of acquired business							$ 410	529
Indefinite-lived intangible assets acquired		1
Finite-lived intangible assets acquired		$ 46
Weighted average useful life of finite-lived intangible assets acquired		7 years
Goodwill		$ 16
Selective Insurance Company of America Self-Insured Pooled Group [Member]
Acquisitions and Sale of Businesses [Line Items]
Payments to acquire business			$ 8
In-force gross written premiums of acquired book of business			$ 38
Gross written premiums of acquired business								34
Net written premiums of acquired business								16
Maximum [Member] | Summit Holding Southeast, Inc [Member]
Acquisitions and Sale of Businesses [Line Items]
Expenses related to the acquisition of business (are less than)		$ 1
AFG [Member]
Acquisitions and Sale of Businesses [Line Items]
Capital contributions to acquired business								$ 27	$ 431	$ 38
AFG [Member] | Run-off long-term care insurance business [Member]
Acquisitions and Sale of Businesses [Line Items]
Number of long-term care insurance policies | policy				1,700				1,700
Long-term care insurance reserves				$ 34				$ 34
Reduced estimated tax payments [Member] | Run-off long-term care insurance business [Member]
Acquisitions and Sale of Businesses [Line Items]
Expected tax benefits proceeds from the sale of business								65
Carryback of the tax-basis capital loss [Member] | Run-off long-term care insurance business [Member]
Acquisitions and Sale of Businesses [Line Items]
Expected tax benefits proceeds from the sale of business								$ 32
Property and Casualty Group [Member]
Acquisitions and Sale of Businesses [Line Items]
Realized gains (losses) on subsidiaries					$ 5