Fair Value Measurements (Quantitative Disclosures at Fair Value Assets) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments Net Asset Value		$ 83,600	$ 46,800
Fixed Income Securities, Available-for-sale, at fair value		6,709,976	6,455,928
Equity securities		335,537	310,367
Short-term investments		447,863	409,852
Available-for-sale Securities [Member] | Net Investment Income Earned [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income		(270)
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		6,709,976	6,455,928
Equity securities		335,537	310,367
Short-term investments		447,863	409,852
Total Assets		7,493,376	7,176,147
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		130,458	116,140
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		15,860	18,366
Obligations of States and Political Subdivisions [Member] | Available-for-sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases		0	0
Sales		0	0
Issuances		0	0
Settlements		0	0
Transfers into Level 3		5,101	2,890
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		0	0
Total net (losses) gains for the period included in OCI		(239)	4
Period change unrealized gains (losses) in earnings		(11)	0
Period change unrealized gains (losses) in OCI		(239)	4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		7,745	2,894	$ 0
Obligations of States and Political Subdivisions [Member] | Available-for-sale Securities [Member] | Net Investment Income Earned [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income		0	0
Obligations of States and Political Subdivisions [Member] | Available-for-sale Securities [Member] | Gain (Loss) on Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income		(11)	0
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		1,189,308	1,247,137
Corporate Securities [Member] | Available-for-sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income		27
Purchases		64,813	46,150
Sales		0	0
Issuances		0	0
Settlements		(544)	(283)
Transfers into Level 3		981	9,592
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		23,436	0
Total net (losses) gains for the period included in OCI		1,636	(785)
Period change unrealized gains (losses) in earnings		(50)	(1,046)
Period change unrealized gains (losses) in OCI		1,636	(785)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		114,127	70,700	17,051
Corporate Securities [Member] | Available-for-sale Securities [Member] | Net Investment Income Earned [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income		(50)	21
Corporate Securities [Member] | Available-for-sale Securities [Member] | Gain (Loss) on Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income			(1,046)
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		2,573,603	2,328,052
Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Available-for-sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income		16
Purchases		76,731	25,785
Sales		0	0
Issuances		0	0
Settlements		(5,161)	(2,638)
Transfers into Level 3		11,344	31,520
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		13,662	(16,978)
Total net (losses) gains for the period included in OCI		(520)	1,883
Period change unrealized gains (losses) in earnings		(214)	(237)
Period change unrealized gains (losses) in OCI		(520)	1,883
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		124,909	56,375	17,034
Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Available-for-sale Securities [Member] | Net Investment Income Earned [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income		(214)	6
Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Available-for-sale Securities [Member] | Gain (Loss) on Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income			(237)
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		1,350,814	1,026,551
Residential Mortgage Backed Securities [Member] | Available-for-sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income		0
Purchases		249
Sales		0
Issuances		0
Settlements		(4)
Transfers into Level 3		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		0
Total net (losses) gains for the period included in OCI		0
Period change unrealized gains (losses) in earnings		0
Period change unrealized gains (losses) in OCI		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		245	0
Residential Mortgage Backed Securities [Member] | Available-for-sale Securities [Member] | Net Investment Income Earned [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income		0
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		776,252	667,894
Commercial Mortgage-backed Securities (CMBS) [Member] | Available-for-sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income		19
Purchases		98
Sales		0
Issuances		0
Settlements		(52)
Transfers into Level 3		4,382
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		0
Total net (losses) gains for the period included in OCI		(196)
Period change unrealized gains (losses) in earnings		5
Period change unrealized gains (losses) in OCI		(196)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		4,256	0
Commercial Mortgage-backed Securities (CMBS) [Member] | Available-for-sale Securities [Member] | Net Investment Income Earned [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income		5
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		673,681	1,051,788
Common Stock [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	[1]	333,449	308,632
Nonredeemable Preferred Stock [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities		2,088	1,735
Investments [Member] | Available-for-sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchases		141,891	71,935
Sales		0	0
Issuances		0	0
Settlements		(5,761)	(2,921)
Transfers into Level 3		21,808	44,002
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		37,098	(16,978)
Total net (losses) gains for the period included in OCI		681	1,102
Period change unrealized gains (losses) in earnings		(270)	(1,283)
Period change unrealized gains (losses) in OCI		681	1,102
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		251,282	129,969	$ 34,085
Investments [Member] | Available-for-sale Securities [Member] | Net Investment Income Earned [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income		62	27
Investments [Member] | Available-for-sale Securities [Member] | Gain (Loss) on Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total net (losses) gains for the period included in net income			(1,283)
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		60,615	40,960
Equity securities		251,934	263,581
Short-term investments		442,723	405,400
Total Assets		755,272	709,941
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		60,615	40,960
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	0
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]	249,846	261,846
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		2,088	1,735
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		6,398,079	6,284,999
Equity securities		0	0
Short-term investments		5,140	4,452
Total Assets		6,403,219	6,289,451
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		69,843	75,180
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		15,860	18,366
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		1,181,563	1,244,243
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		2,459,476	2,257,352
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		1,225,905	970,176
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		776,007	667,894
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		669,425	1,051,788
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		251,282	129,969
Equity securities		0	0
Short-term investments		0	0
Total Assets		251,282	129,969
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		7,745	2,894
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		114,127	70,700
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		124,909	56,375
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		245	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		4,256	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 $83.6 million and $46.8 million at December 31, 2021 and December 31, 2020, 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.