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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($) $ in Thousands		Sep. 30, 2022	Dec. 31, 2021
AFS fixed income securities		$ 6,472,100	$ 6,709,976
Equity securities		183,868	335,537
Short-term investments		269,302	447,863
Investments measured at fair value using the net asset value per share.	[1]	101,100	83,600
Fair Value, Recurring [Member]
AFS fixed income securities		6,472,100	6,709,976
Equity securities		183,868	335,537
Short-term investments		269,302	447,863
Total assets measured at fair value		6,925,270	7,493,376
US Government and Government Agencies [Member]
AFS fixed income securities		148,231	130,458
US Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		148,231	130,458
Foreign Government [Member]
AFS fixed income securities		13,780	15,860
Foreign Government [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		13,780	15,860
Obligations of States and Political Subdivisions [Member]
AFS fixed income securities		967,630	1,189,308
Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		967,630	1,189,308
Corporate Securities [Member]
AFS fixed income securities		2,264,636	2,573,603
Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		2,264,636	2,573,603
Collateralized Loan Obligations and Other Asset Backed Securities [Member]
AFS fixed income securities		1,467,423	1,350,814
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		1,467,423	1,350,814
Residential Mortgage Backed Securities [Member]
AFS fixed income securities		1,016,057	776,252
Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		1,016,057	776,252
Commercial Mortgage Backed Securities [Member]
AFS fixed income securities		594,343	673,681
Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		594,343	673,681
Common Stock [Member] | Fair Value, Recurring [Member]
Equity securities	[1]	182,138	333,449
Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity securities		1,730	2,088
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		75,858	60,615
Equity securities		82,759	251,934
Short-term investments		268,982	442,723
Total assets measured at fair value		427,599	755,272
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | US Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		75,858	60,615
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
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]
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]
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]
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]
AFS fixed income securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
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]
Equity securities	[1]	81,029	249,846
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity securities		1,730	2,088
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		6,097,662	6,398,079
Equity securities		0	0
Short-term investments		320	5,140
Total assets measured at fair value		6,097,982	6,403,219
Significant Other Observable Inputs (Level 2) [Member] | US Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		72,373	69,843
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		13,780	15,860
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		960,919	1,181,563
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		2,098,812	2,459,476
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		1,341,751	1,225,905
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		1,016,057	776,007
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		593,970	669,425
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Equity securities	[1]	0	0
Significant Other Observable Inputs (Level 2) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		298,580	251,282
Equity securities		0	0
Short-term investments		0	0
Total assets measured at fair value		298,580	251,282
Significant Unobservable Inputs (Level 3) [Member] | US Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		6,711	7,745
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		165,824	114,127
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		125,672	124,909
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		0	245
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS fixed income securities		373	4,256
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Equity securities	[1]	0	0
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity securities		$ 0	$ 0
[1] Investments amounting to $101.1 million at September 30, 2022, and $83.6 million at December 31, 2021, 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 in this table are intended to permit reconciliation of the fair value hierarchy to total assets measured at fair value.

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