Business Combinations	9 Months Ended
Sep. 30, 2011
Business Combinations [Abstract]
Business Combinations	NOTE 16. Business Combinations In August 2011, one of our Insurance Subsidiaries, Selective Insurance Company of America ("SICA"), purchased the renewal rights to the commercial excess and surplus ("E & S") lines business written under contract binding authority by Alterra Excess & Surplus Insurance Company ("Alterra"). This business generated gross premiums written of approximately $77 million in 2010. If all of this business had been written by SICA in 2010, our NPW would have increased by approximately 6%. Considering the size of the book of business, we do not believe it would be meaningful to provide historical proforma financial information regarding this transaction. To provide a legal entity licensed to write E & S lines of business, in September 2011, the Parent entered into an agreement to purchase Montpelier U.S. Insurance Company ("MUSIC"), a wholly-owned E & S lines subsidiary of Montpelier Re Holdings Ltd. ("Montpelier Re"). Under the terms of the agreement, the Parent agreed to acquire all of the issued and outstanding shares of common stock of MUSIC. This acquisition provides a nationally-licensed platform that will allow us to write contract binding authority E & S business. Upon closing, which is expected to occur in the fourth quarter of 2011, Montpelier Reinsurance Ltd., a wholly-owned subsidiary of Montpelier Re, and MUSIC will enter into several reinsurance agreements that will indemnify the Parent for any adverse loss development and any other obligations of MUSIC that relate to business written prior to the date of the acquisition. These reinsurance agreements will also provide that the Parent reimburse Montpelier Reinsurance Ltd. for any favorable loss development that is recognized that relates to business written prior to the date of the acquisition. These reinsurance obligations will be collateralized through a trust arrangement. Based on MUSIC's net asset value at June 30, 2011, the transaction is valued at approximately $55 million.

Business Combinations

9 Months Ended

Sep. 30, 2011

NOTE 16. Business Combinations

In August 2011, one of our Insurance Subsidiaries, Selective Insurance Company of America ("SICA"), purchased the renewal rights to the commercial excess and surplus ("E & S") lines business written under contract binding authority by Alterra Excess & Surplus Insurance Company ("Alterra"). This business generated gross premiums written of approximately $77 million in 2010. If all of this business had been written by SICA in 2010, our NPW would have increased by approximately 6%. Considering the size of the book of business, we do not believe it would be meaningful to provide historical proforma financial information regarding this transaction.

To provide a legal entity licensed to write E & S lines of business, in September 2011, the Parent entered into an agreement to purchase Montpelier U.S. Insurance Company ("MUSIC"), a wholly-owned E & S lines subsidiary of Montpelier Re Holdings Ltd. ("Montpelier Re"). Under the terms of the agreement, the Parent agreed to acquire all of the issued and outstanding shares of common stock of MUSIC.

This acquisition provides a nationally-licensed platform that will allow us to write contract binding authority E & S business. Upon closing, which is expected to occur in the fourth quarter of 2011, Montpelier Reinsurance Ltd., a wholly-owned subsidiary of Montpelier Re, and MUSIC will enter into several reinsurance agreements that will indemnify the Parent for any adverse loss development and any other obligations of MUSIC that relate to business written prior to the date of the acquisition. These reinsurance agreements will also provide that the Parent reimburse Montpelier Reinsurance Ltd. for any favorable loss development that is recognized that relates to business written prior to the date of the acquisition. These reinsurance obligations will be collateralized through a trust arrangement. Based on MUSIC's net asset value at June 30, 2011, the transaction is valued at approximately $55 million.