Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Available-for-sale Secuities	$ 3,465,981		$ 3,296,013
Equity securities, AFS	174,745		151,382
Short-term investments	163,440		214,479
Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	3,465,981		3,296,013
Equity securities, AFS	174,745		151,382
Short-term investments	163,440		214,479
Receivable for proceeds related to sale of Selective HR			2,705
Total assets	3,804,166		3,664,579
Fair Value [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	245,664		259,092
Fair Value [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	30,163		30,229
Fair Value [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	849,946		818,024
Fair Value [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	1,532,753		1,450,247
Fair Value [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	171,943		128,640
Fair Value [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	146,540		137,119
Fair Value [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	488,972		472,662
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	104,507	[1]	115,861	[1]
Equity securities, AFS	167,203	[1]	147,775	[1]
Short-term investments	163,440	[1]	214,479	[1]
Receivable for proceeds related to sale of Selective HR			0	[1]
Total assets	435,150	[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	104,507	[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	3,327,183	[1]	3,144,187	[1]
Equity securities, AFS	4,642	[1]	0	[1]
Short-term investments	0	[1]	0	[1]
Receivable for proceeds related to sale of Selective HR			0	[1]
Total assets	3,331,825	[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	122,185	[1]	123,442	[1]
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	30,163	[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	849,946	[1]	818,024	[1]
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	1,529,851	[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	165,887	[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	140,179	[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	488,972	[1]	472,662	[1]
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	34,291		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	37,191		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	18,972		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	2,902		2,946
Significant Unobservable Inputs (Level 3) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	6,056		6,068
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale Secuities	6,361		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.