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Fair Value Measurements (Quantitative Disclosures at Fair Value Assets) (Details) - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments Net Asset Value		$ 46,800	$ 37,800
Fixed Income Securities, Available-for-sale, at fair value		6,455,928	6,095,620
Equity securities		310,367	72,937
Short-term investments		409,852	282,490
Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		6,455,928	6,095,620
Equity securities		310,367	72,937
Short-term investments		409,852	282,490
Total Assets		7,176,147	6,451,047
U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		116,140	116,186
Foreign Government [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		18,366	18,542
Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		1,247,137	1,230,090
Corporate Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		2,328,052	1,947,477
Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		1,026,551	793,012
Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		1,051,788	538,344
Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		667,894	1,451,969
Common Stock [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	[1]	308,632	69,900
Nonredeemable Preferred Stock [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities		1,735	3,037
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		40,960	44,718
Equity securities		263,581	35,182
Short-term investments		405,400	265,306
Total Assets		709,941	345,206
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		40,960	41,083
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		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, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		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, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		0	3,635
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		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, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	[1]	261,846	32,145
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Nonredeemable Preferred Stock [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities		1,735	3,037
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		6,284,999	6,016,817
Equity securities		0	0
Short-term investments		4,452	17,184
Total Assets		6,289,451	6,034,001
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		75,180	75,103
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		18,366	18,542
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		1,244,243	1,230,090
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		2,257,352	1,930,426
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		970,176	772,343
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		1,051,788	538,344
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		667,894	1,451,969
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	[1]	0	0
Significant Other Observable Inputs (Level 2) [Member] | Nonredeemable Preferred Stock [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		129,969	34,085
Equity securities		0	0
Short-term investments		0	0
Total Assets		129,969	34,085
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		0	0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		0	0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		2,894	0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		70,700	17,051
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		56,375	17,034
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		0	0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Income Securities, Available-for-sale, at fair value		0	0
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	[1]	0	0
Significant Unobservable Inputs (Level 3) [Member] | Nonredeemable Preferred Stock [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities		$ 0	$ 0
[1] Investments amounting to $46.8 million and $37.8 million at December 31, 2020 and December 31, 2019, 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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