Acquisitions and Sale of Businesses (Details Textual) - USD ($) $ in Millions			3 Months Ended			9 Months Ended			12 Months Ended
	Apr. 01, 2014	Mar. 27, 2014	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2014
Acquisitions and Sale of Businesses [Line Items]
Payments to acquire business						$ 0		$ 267
Run-off long-term care insurance business [Member]
Acquisitions and Sale of Businesses [Line Items]
Sale proceeds				$ 7
Additional sale proceeds based on the release of certain statutory liabilities of the legal entities sold						$ 13
Carrying value of the assets and liabilities of business sold as a percentage of total assets and liabilities					4.00%	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 written premiums of acquired business							$ 410		$ 539
Net earned premiums of acquired business			$ 128			$ 386
Selective Insurance Company of America Self-Insured Pooled Group [Member]
Acquisitions and Sale of Businesses [Line Items]
Payments to acquire business		$ 8
Net written premiums of acquired business									23
In-force gross written premiums of acquired book of business		$ 38
Gross written premiums of acquired business									$ 33
Minimum [Member] | Run-off long-term care insurance business [Member]
Acquisitions and Sale of Businesses [Line Items]
Expected after-tax cash proceeds from the sale of business						105
Maximum [Member] | Run-off long-term care insurance business [Member]
Acquisitions and Sale of Businesses [Line Items]
Expected after-tax cash proceeds from the sale of business						$ 115
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