Summary Of Investments Other Than Investments In Related Parties (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 3,600,120	$ 3,502,880
Fair value	3,732,728	3,547,560
Amounts in the balance sheet	3,732,728	3,547,560
Fixed Maturity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,823,230	2,795,410
Fair value	2,892,777	2,814,553
Amounts in the balance sheet	2,892,777	2,814,553
Fixed Maturity Securities [Member] \| U.S. Government Bonds And Agencies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	13,355	12,288
Fair value	13,236	12,275
Amounts in the balance sheet	13,236	12,275
Fixed Maturity Securities [Member] \| Municipal Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,490,150	2,432,181
Fair value	2,556,532	2,449,292
Amounts in the balance sheet	2,556,532	2,449,292
Fixed Maturity Securities [Member] \| Collateralized Mortgage Backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	26,454	39,082
Fair value	27,165	39,777
Amounts in the balance sheet	27,165	39,777
Fixed Maturity Securities [Member] \| Corporate Debt Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	136,013	189,780
Fair value	137,542	189,688
Amounts in the balance sheet	137,542	189,688
Fixed Maturity Securities [Member] \| Collateralized Debt Obligations [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	104,323	85,429
Fair value	105,202	86,525
Amounts in the balance sheet	105,202	86,525
Fixed Maturity Securities [Member] \| Other asset-backed securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	52,935	36,650
Fair value	53,100	36,996
Amounts in the balance sheet	53,100	36,996
Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	474,197	331,770
Fair value	537,240	357,327
Amounts in the balance sheet	537,240	357,327
Equity Securities [Member] \| Common Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	368,619	286,503
Fair value	429,367	316,450
Amounts in the balance sheet	429,367	316,450
Equity Securities [Member] \| Nonredeemable Preferred Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	34,429	32,436
Fair value	34,869	31,809
Amounts in the balance sheet	34,869	31,809
Equity Securities [Member] \| Private Equity Funds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,481	12,831
Fair value	1,481	9,068
Amounts in the balance sheet	1,481	9,068
Equity Securities [Member] \| Private Equity Funds, Net Asset Value [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	69,668
Fair value	71,523
Amounts in the balance sheet	71,523
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	302,693	375,700
Fair value	302,711	375,680
Amounts in the balance sheet	$ 302,711	$ 375,680

This element represents the carrying (reported) amount of the entity's investments as of the balance sheet date. This amount may represent the individual investment's or group of similar investments' original cost, fair value, or such other amount at which the investment is shown in the entity's balance sheet.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611379-123010

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This item represents the original cost of the entity's investments including investments in equity securities and, as to fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums or accrual of discounts.

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