Fair Value Measurement (Schedule Of Assets And Liabilities Measured At Fair Value On A Recurring Basis Valuation Techniques) (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	$ 3,732,917,000	$ 3,548,227,000
Liabilities, fair value	1,735,000	785,000
Notes payable	371,236,000	320,000,000
Money Market Instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	366,402,000	375,680,000
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	772,235,000	684,172,000
Liabilities, fair value	146,000	20,000
Level 1 [Member] | Money Market Instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	347,509,000	355,447,000
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	2,882,987,000	2,854,987,000
Liabilities, fair value	1,589,000	765,000
Level 2 [Member] | Money Market Instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	18,893,000	20,233,000
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	8,684,000	9,068,000
Liabilities, fair value	0	0
Level 3 [Member] | Money Market Instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Note Receivable [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	5,585,000
Note Receivable [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0
Note Receivable [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	5,585,000
Note Receivable [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0
Fixed maturity securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	2,841,736,000	2,814,553,000
Fixed maturity securities [Member] | U.S. Government Bonds and Agencies [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	13,892,000	12,275,000
Fixed maturity securities [Member] | Municipal Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	2,483,750,000	2,449,292,000
Fixed maturity securities [Member] | Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	32,726,000	39,777,000
Fixed maturity securities [Member] | Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	161,883,000	189,688,000
Fixed maturity securities [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	94,325,000	86,525,000
Fixed maturity securities [Member] | Other Asset-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	55,160,000	36,996,000
Fixed maturity securities [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	13,892,000	12,275,000
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	13,892,000	12,275,000
Fixed maturity securities [Member] | Level 1 [Member] | Municipal Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Fixed maturity securities [Member] | Level 1 [Member] | Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Fixed maturity securities [Member] | Level 1 [Member] | Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Fixed maturity securities [Member] | Level 1 [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
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	0	0
Fixed maturity securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	2,827,844,000	2,802,278,000
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	0	0
Fixed maturity securities [Member] | Level 2 [Member] | Municipal Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	2,483,750,000	2,449,292,000
Fixed maturity securities [Member] | Level 2 [Member] | Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	32,726,000	39,777,000
Fixed maturity securities [Member] | Level 2 [Member] | Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	161,883,000	189,688,000
Fixed maturity securities [Member] | Level 2 [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	94,325,000	86,525,000
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	55,160,000	36,996,000
Fixed maturity securities [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
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	0	0
Fixed maturity securities [Member] | Level 3 [Member] | Municipal Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Fixed maturity securities [Member] | Level 3 [Member] | Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Fixed maturity securities [Member] | Level 3 [Member] | Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Fixed maturity securities [Member] | Level 3 [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
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	0	0
Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	519,194,000	357,327,000
Equity Securities [Member] | Common Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	410,834,000	316,450,000
Equity Securities [Member] | Nonredeemable Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	30,665,000	31,809,000
Equity Securities [Member] | Private Equity Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	8,684,000	9,068,000
Equity Securities [Member] | Private Equity Funds, Net Asset Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	69,011,000
Equity Securities [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	410,834,000	316,450,000
Equity Securities [Member] | Level 1 [Member] | Common Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	410,834,000	316,450,000
Equity Securities [Member] | Level 1 [Member] | Nonredeemable Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Equity Securities [Member] | Level 1 [Member] | Private Equity Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Equity Securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	30,665,000	31,809,000
Equity Securities [Member] | Level 2 [Member] | Common Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Equity Securities [Member] | Level 2 [Member] | Nonredeemable Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	30,665,000	31,809,000
Equity Securities [Member] | Level 2 [Member] | Private Equity Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Equity Securities [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	8,684,000	9,068,000
Equity Securities [Member] | Level 3 [Member] | Common Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Equity Securities [Member] | Level 3 [Member] | Nonredeemable Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Equity Securities [Member] | Level 3 [Member] | Private Equity Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	8,684,000	9,068,000
Short-term Investments [Member] | Short-term Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	48,889,000	90,626,000
Short-term Investments [Member] | Level 1 [Member] | Short-term Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	29,996,000	70,393,000
Short-term Investments [Member] | Level 2 [Member] | Short-term Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	18,893,000	20,233,000
Short-term Investments [Member] | Level 3 [Member] | Short-term Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Money Market Instruments [Member] | Money Market Instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	317,513,000	285,054,000
Money Market Instruments [Member] | Level 1 [Member] | Money Market Instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	317,513,000	285,054,000
Money Market Instruments [Member] | Level 2 [Member] | Money Market Instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Money Market Instruments [Member] | Level 3 [Member] | Money Market Instruments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value	0	0
Total Return Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value		667,000
Liabilities, fair value	1,589,000	765,000
Total Return Swap [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value		0
Liabilities, fair value	0	0
Total Return Swap [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value		667,000
Liabilities, fair value	1,589,000	765,000
Total Return Swap [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value		0
Liabilities, fair value	0	0
Equity contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	146,000	20,000
Equity contracts [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	146,000	20,000
Equity contracts [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	0	0
Equity contracts [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	0	0
Unsecured debt [Member] | Borrowings [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	385,733,000	180,000,000
Notes payable	371,236,000	180,000,000
Unsecured debt [Member] | Borrowings [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	0	0
Unsecured debt [Member] | Borrowings [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	385,733,000	180,000,000
Unsecured debt [Member] | Borrowings [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	0	0
Secured Debt [Member] | Borrowings [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value	$ 0	140,000,000
Notes payable		140,000,000
Secured Debt [Member] | Borrowings [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value		0
Secured Debt [Member] | Borrowings [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value		140,000,000
Secured Debt [Member] | Borrowings [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities, fair value		$ 0