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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Available-for-sale Secuities	$ 3,633,432		$ 3,296,013
Equity securities, AFS	180,506		151,382
Short-term investments	172,087		214,479
Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	3,633,432		3,296,013
Equity securities, AFS	180,506		151,382
Short-term investments	172,087		214,479
Receivable for proceeds related to sale of Selective HR			2,705
Total assets	3,986,025		3,664,579
Fair Value [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	180,786		259,092
Fair Value [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	29,709		30,229
Fair Value [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	929,498		818,024
Fair Value [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	1,663,151		1,450,247
Fair Value [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	150,766		128,640
Fair Value [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	151,948		137,119
Fair Value [Member] | Residential Mortgage Backed Securities (RMBS) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	527,574		472,662
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	50,871	[1]	115,861	[1]
Equity securities, AFS	177,606	[1]	147,775	[1]
Short-term investments	172,087	[1]	214,479	[1]
Receivable for proceeds related to sale of Selective HR			0	[1]
Total assets	400,564	[1]	478,115	[1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	50,871	[1]	115,861	[1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	0	[1]	0	[1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	0	[1]	0	[1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	0	[1]	0	[1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	0	[1]	0	[1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	0	[1]	0	[1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities (RMBS) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	0	[1]	0	[1]
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	3,574,990	[1]	3,144,187	[1]
Equity securities, AFS	0	[1]	0	[1]
Short-term investments	0	[1]	0	[1]
Receivable for proceeds related to sale of Selective HR			0	[1]
Total assets	3,574,990	[1]	3,144,187	[1]
Significant Other Observable Inputs (Level 2) [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	129,915	[1]	123,442	[1]
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	29,709	[1]	30,229	[1]
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	929,498	[1]	818,024	[1]
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	1,663,151	[1]	1,447,301	[1]
Significant Other Observable Inputs (Level 2) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	144,804	[1]	122,572	[1]
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	150,339	[1]	129,957	[1]
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities (RMBS) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	527,574	[1]	472,662	[1]
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	7,571		35,965
Equity securities, AFS	2,900		3,607
Short-term investments	0		0
Receivable for proceeds related to sale of Selective HR			2,705
Total assets	10,471		42,277
Significant Unobservable Inputs (Level 3) [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	0		19,789
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	0		2,946
Significant Unobservable Inputs (Level 3) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	5,962		6,068
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	1,609		7,162
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities (RMBS) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	$ 0		$ 0
[1] 1 There were no transfers of securities between Level 1 and Level 2.

X

- Definition

The estimated proceeds associated with the sale of a subsidiary, net of payments received.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Amount of equity securities categorized neither as held-to-maturity nor as trading.

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Other investments not otherwise specified in the taxonomy that will be realized in a short period of time, usually less than one year or the normal operating cycle, whichever is longer.

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