Fair Value Measurement (Schedule Of Assets And Liabilities Measured At Fair Value On A Recurring Basis Valuation Techniques) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Fair Value Measurement [Line Items]
Total assets at fair value	$ 3,119,246	$ 3,062,421
Total liabilities at fair value	951	1,325
Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	594,594	600,692
Total liabilities at fair value	405	655
Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	2,461,159	2,404,218
Total liabilities at fair value	546	670
Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	63,493	57,511
Total liabilities at fair value	0	0
Partnership Interest In A Private Credit Fund [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	10,510	10,008
Partnership Interest In A Private Credit Fund [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Partnership Interest In A Private Credit Fund [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Partnership Interest In A Private Credit Fund [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	10,510	10,008
Short-Term Bonds [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	31,754	9,011
Short-Term Bonds [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Short-Term Bonds [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	31,754	9,011
Short-Term Bonds [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Money Market Instruments [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	195,442	227,433
Money Market Instruments [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	195,442	227,433
Money Market Instruments [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Money Market Instruments [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Equity Contracts [Member]
Fair Value Measurement [Line Items]
Total liabilities at fair value	405	655
Equity Contracts [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total liabilities at fair value	405	655
Equity Contracts [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total liabilities at fair value	0	0
Equity Contracts [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total liabilities at fair value	0	0
Interest Rate Swap Agreements [Member]
Fair Value Measurement [Line Items]
Total liabilities at fair value	546	670
Interest Rate Swap Agreements [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total liabilities at fair value	0	0
Interest Rate Swap Agreements [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total liabilities at fair value	546	670
Interest Rate Swap Agreements [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total liabilities at fair value	0	0
Fixed Maturity Securities [Member] \| U.S. Government Bonds And Agencies [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	14,181	14,298
Fixed Maturity Securities [Member] \| U.S. Government Bonds And Agencies [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	14,181	14,298
Fixed Maturity Securities [Member] \| U.S. Government Bonds And Agencies [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Fixed Maturity Securities [Member] \| U.S. Government Bonds And Agencies [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Fixed Maturity Securities [Member] \| Municipal Securities [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	2,289,120	2,271,275
Fixed Maturity Securities [Member] \| Municipal Securities [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Fixed Maturity Securities [Member] \| Municipal Securities [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	2,289,120	2,271,275
Fixed Maturity Securities [Member] \| Municipal Securities [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Fixed Maturity Securities [Member] \| Mortgage-Backed Securities [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	34,914	37,371
Fixed Maturity Securities [Member] \| Mortgage-Backed Securities [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Fixed Maturity Securities [Member] \| Mortgage-Backed Securities [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	34,914	37,371
Fixed Maturity Securities [Member] \| Mortgage-Backed Securities [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Fixed Maturity Securities [Member] \| Corporate Securities [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	94,396	75,142
Fixed Maturity Securities [Member] \| Corporate Securities [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Fixed Maturity Securities [Member] \| Corporate Securities [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	94,396	75,142
Fixed Maturity Securities [Member] \| Corporate Securities [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Fixed Maturity Securities [Member] \| Collateralized Debt Obligations [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	52,983	47,503
Fixed Maturity Securities [Member] \| Collateralized Debt Obligations [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Fixed Maturity Securities [Member] \| Collateralized Debt Obligations [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Fixed Maturity Securities [Member] \| Collateralized Debt Obligations [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	52,983	47,503
Equity Securities [Member] \| Public Utilities [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	29,369	26,342
Equity Securities [Member] \| Public Utilities [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	29,369	26,342
Equity Securities [Member] \| Public Utilities [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Equity Securities [Member] \| Public Utilities [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Equity Securities [Member] \| Banks, Trusts And Insurance Companies [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	19,431	16,027
Equity Securities [Member] \| Banks, Trusts And Insurance Companies [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	19,431	16,027
Equity Securities [Member] \| Banks, Trusts And Insurance Companies [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Equity Securities [Member] \| Banks, Trusts And Insurance Companies [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Equity Securities [Member] \| Industrial And Other [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	336,171	316,592
Equity Securities [Member] \| Industrial And Other [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	336,171	316,592
Equity Securities [Member] \| Industrial And Other [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Equity Securities [Member] \| Industrial And Other [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Equity Securities [Member] \| Non-Redeemable Preferred Stock [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	10,975	11,419
Equity Securities [Member] \| Non-Redeemable Preferred Stock [Member] \| Level 1 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	0	0
Equity Securities [Member] \| Non-Redeemable Preferred Stock [Member] \| Level 2 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	10,975	11,419
Equity Securities [Member] \| Non-Redeemable Preferred Stock [Member] \| Level 3 [Member]
Fair Value Measurement [Line Items]
Total assets at fair value	$ 0	$ 0