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Acquisitions (Details Textual) (USD $) In Millions, unless otherwise specified | 9 Months Ended |  | 0 Months Ended | 6 Months Ended |  | 12 Months Ended |
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Sep. 30, 2014 | Sep. 30, 2013 | Dec. 31, 2013 | Mar. 27, 2014 Selective Insurance Company of America Self-Insured Pooled Group [Member] | Apr. 01, 2014 Summit Holding Southeast, Inc [Member] | Sep. 30, 2014 Summit Holding Southeast, Inc [Member] | Apr. 01, 2014 Summit Holding Southeast, Inc [Member] | Dec. 31, 2013 Minimum [Member] Summit Holding Southeast, Inc [Member] | Apr. 01, 2014 Maximum [Member] Summit Holding Southeast, Inc [Member]
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Payments to acquire business | $ 267 | $ 0 |  | $ 8 | $ 259 |  |  |  |
In-force gross written premiums of acquired book of business |  |  |  | 38 |  |  |  |  |
Capital contributions to acquired business |  |  |  |  | 140 |  |  |  |
Total capital investment in acquired business |  |  |  |  | 399 |  |  |  |
Net written premiums of acquired business |  |  |  |  |  |  |  | 500 |
Net earned premiums of acquired business |  |  |  |  |  | 268 |  |  |
Expenses related to the acquisition of business (are less than) |  |  |  |  |  |  |  |  | 1
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 | $ 201 |  | $ 185 |  |  |  | $ 16 |  |

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