Fair Value Measurement (Schedule Of Assets And Liabilities Measured At Fair Value On A Recurring Basis Valuation Techniques) (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	$ 371,635,000	$ 371,335,000
Unsecured debt [Member] | Borrowings [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	371,635,000	371,335,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	3,909,578,000	3,738,293,000
Liabilities, fair value	1,550,000	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	1,400,000	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	836,714,000	743,294,000
Liabilities, fair value	589,000	123,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	3,001,954,000	2,921,995,000
Liabilities, fair value	961,000	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	1,446,000	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	2,952,871,000	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	14,105,000	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	2,590,847,000	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	31,513,000	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	119,292,000	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	155,796,000	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	41,318,000	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	14,105,000	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	14,105,000	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	2,938,766,000	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	2,590,847,000	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	31,513,000	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	119,292,000	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	155,796,000	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	41,318,000	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	688,387,000	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	583,851,000	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	33,626,000	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	1,446,000	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	69,464,000	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	583,851,000	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	583,851,000	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	33,626,000	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	33,626,000	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	1,446,000	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	1,446,000	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	262,806,000	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	55,496,000	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	207,310,000	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	238,758,000	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	31,448,000	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	207,310,000	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	24,048,000	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	24,048,000	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	5,514,000	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	5,514,000	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	961,000	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	961,000	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	589,000	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	589,000	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	361,118,000	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	361,118,000	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