Fair Value Measurement (Schedule Of Assets And Liabilities Measured At Fair Value On A Recurring Basis Valuation Techniques) (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	$ 371,535,000	$ 371,335,000
Unsecured debt [Member] | Borrowings [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	371,535,000	371,335,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	3,839,672,000	3,738,293,000
Liabilities, fair value	2,173,000	1,323,000
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	1,500,000	1,500,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	768,707,000	743,294,000
Liabilities, fair value	780,000	123,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	2,998,476,000	2,921,995,000
Liabilities, fair value	1,393,000	1,200,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	1,450,000	1,481,000
Liabilities, fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	2,949,236,000	2,892,777,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | U.S. Government Bonds and Agencies [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	13,333,000	13,236,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Municipal Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	2,616,770,000	2,556,532,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	32,214,000	27,165,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	116,401,000	137,542,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	135,453,000	105,202,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Other Asset-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	35,065,000	53,100,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	13,333,000	13,236,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 1 [Member] | U.S. Government Bonds and Agencies [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	13,333,000	13,236,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 1 [Member] | Municipal Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 1 [Member] | Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 1 [Member] | Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 1 [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 1 [Member] | Other Asset-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	2,935,903,000	2,879,541,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 2 [Member] | U.S. Government Bonds and Agencies [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 2 [Member] | Municipal Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	2,616,770,000	2,556,532,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 2 [Member] | Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	32,214,000	27,165,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 2 [Member] | Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	116,401,000	137,542,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 2 [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	135,453,000	105,202,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 2 [Member] | Other Asset-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	35,065,000	53,100,000
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 3 [Member] | U.S. Government Bonds and Agencies [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 3 [Member] | Municipal Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 3 [Member] | Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 3 [Member] | Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 3 [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Fixed maturity securities [Member] | Level 3 [Member] | Other Asset-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	614,336,000	537,240,000
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Common Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	507,504,000	429,367,000
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Nonredeemable Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	34,343,000	34,869,000
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Private Equity Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	1,450,000	1,481,000
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Private Equity Funds Net Asset Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	71,039,000	71,523,000
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	507,504,000	429,367,000
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Level 1 [Member] | Common Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	507,504,000	429,367,000
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Level 1 [Member] | Nonredeemable Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Level 1 [Member] | Private Equity Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	34,343,000	34,869,000
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Level 2 [Member] | Common Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Level 2 [Member] | Nonredeemable Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	34,343,000	34,869,000
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Level 2 [Member] | Private Equity Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	1,450,000	1,481,000
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Level 3 [Member] | Common Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Level 3 [Member] | Nonredeemable Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Level 3 [Member] | Private Equity Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	1,450,000	1,481,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	270,584,000	302,711,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Short-term Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	52,666,000	32,018,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Money Market Instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	217,918,000	270,693,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	247,870,000	300,691,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Level 1 [Member] | Short-term Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	29,952,000	29,998,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Level 1 [Member] | Money Market Instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	217,918,000	270,693,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	22,714,000	2,020,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Level 2 [Member] | Short-term Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	22,714,000	2,020,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Level 2 [Member] | Money Market Instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Level 3 [Member] | Short-term Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Level 3 [Member] | Money Market Instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Other Assets [Member] | Financing Receivable [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	5,516,000	5,565,000
Fair Value, Measurements, Recurring [Member] | Other Assets [Member] | Level 1 [Member] | Financing Receivable [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Other Assets [Member] | Level 2 [Member] | Financing Receivable [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	5,516,000	5,565,000
Fair Value, Measurements, Recurring [Member] | Other Assets [Member] | Level 3 [Member] | Financing Receivable [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	1,393,000	1,200,000
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	0	0
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	1,393,000	1,200,000
Fair Value, Measurements, Recurring [Member] | Total Return Swap [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	0	0
Fair Value, Measurements, Recurring [Member] | Equity contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	780,000	123,000
Fair Value, Measurements, Recurring [Member] | Equity contracts [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	780,000	123,000
Fair Value, Measurements, Recurring [Member] | Equity contracts [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	0	0
Fair Value, Measurements, Recurring [Member] | Equity contracts [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	0	0
Fair Value, Measurements, Recurring [Member] | Unsecured debt [Member] | Borrowings [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	364,736,000	385,583,000
Fair Value, Measurements, Recurring [Member] | Unsecured debt [Member] | Borrowings [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	0	0
Fair Value, Measurements, Recurring [Member] | Unsecured debt [Member] | Borrowings [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	364,736,000	385,583,000
Fair Value, Measurements, Recurring [Member] | Unsecured debt [Member] | Borrowings [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	$ 0	$ 0