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Fair Value Measurements (Quantitative Disclosures of Financial Assets Measured and Recorded at Fair Value) (Details) - USD ($) $ in Thousands		Dec. 31, 2022	Dec. 31, 2021
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		$ 6,612,107	$ 6,709,976
Equity securities		162,000	335,537
Short-term investments		440,456	447,863
Investments measured at fair value using the net asset value per share		103,600	83,600
Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		6,612,107	6,709,976
Equity securities		162,000	335,537
Short-term investments		440,456	447,863
Total assets measured at fair value		7,214,563	7,493,376
U.S. Government and Government Agencies [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		189,239	130,458
U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		189,239	130,458
Foreign Government [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		9,608	15,860
Foreign Government [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		9,608	15,860
Obligations of States and Political Subdivisions [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		918,018	1,189,308
Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		918,018	1,189,308
Corporate Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		2,335,025	2,573,603
Corporate Securities [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		2,335,025	2,573,603
Collateralized Loan Obligations and Other Asset-Backed Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		1,485,973	1,350,814
Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		1,485,973	1,350,814
Residential Mortgage Backed Securities [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		1,059,832	776,252
Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		1,059,832	776,252
Commercial Mortgage-backed Securities (CMBS) [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		614,412	673,681
Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		614,412	673,681
Common Stock [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity securities	[1]	160,355	333,449
Preferred Stock [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity securities		1,645	2,088
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		109,240	60,615
Equity securities		57,491	251,934
Short-term investments		418,199	442,723
Total assets measured at fair value		584,930	755,272
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		109,240	60,615
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity securities	[1]	55,846	249,846
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity securities		1,645	2,088
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		6,154,509	6,398,079
Equity securities		0	0
Short-term investments		22,257	5,140
Total assets measured at fair value		6,176,766	6,403,219
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		79,999	69,843
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		9,608	15,860
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		911,357	1,181,563
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		2,147,045	2,459,476
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		1,332,631	1,225,905
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		1,059,832	776,007
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		614,037	669,425
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity securities	[1]	0	0
Significant Other Observable Inputs (Level 2) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		348,358	251,282
Equity securities		897	0
Short-term investments		0	0
Total assets measured at fair value		349,255	251,282
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		6,661	7,745
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		187,980	114,127
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		153,342	124,909
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		0	245
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
AFS fixed income securities		375	4,256
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity securities	[1]	897	0
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity securities		$ 0	$ 0
[1] Investments amounting to $103.6 million and $83.6 million at December 31, 2022 and December 31, 2021, respectively, were measured at fair value using the net asset value per share (or its practical expedient) and have not been classified in the fair value hierarchy. These investments are not redeemable and the timing of liquidations of the underlying assets is unknown at each reporting period. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to total assets measured at fair value.

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