Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
AFS Fixed Income Securities		$ 6,437,537	$ 6,095,620
Equity Securities		151,824	72,937
Short-term investments		380,890	282,490
Net Asset Value	[1]	46,600	37,800
Fair Value, Recurring [Member]
AFS Fixed Income Securities		6,437,537	6,095,620
Equity Securities		151,824	72,937
Short-term investments		380,890	282,490
Total assets measured at fair value		6,970,251	6,451,047
U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		117,529	116,186
Foreign Government [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		17,008	18,542
Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		1,231,456	1,230,090
Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		2,298,831	1,947,477
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		945,582	793,012
Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		1,182,957	1,451,969
Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		644,174	538,344
Common Stock [Member] | Fair Value, Recurring [Member]
Equity Securities	[1]	150,153	69,900
Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity Securities		1,671	3,037
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		43,287	44,718
Equity Securities		105,274	35,182
Short-term investments		379,317	265,306
Total assets measured at fair value		527,878	345,206
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		43,287	41,083
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	3,635
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]	103,603	32,145
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity Securities		1,671	3,037
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		6,309,040	6,016,817
Equity Securities		0	0
Short-term investments		1,573	17,184
Total assets measured at fair value		6,310,613	6,034,001
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		74,242	75,103
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		17,008	18,542
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		1,228,602	1,230,090
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		2,261,195	1,930,426
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		900,862	772,343
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		1,182,957	1,451,969
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		644,174	538,344
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		85,210	34,085
Equity Securities		0	0
Short-term investments		0	0
Total assets measured at fair value		85,210	34,085
Significant Unobservable Inputs (Level 3) [Member] | U.S. 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		2,854	0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		37,636	17,051
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		44,720	17,034
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
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 $46.6 million at September 30, 2020, and $37.8 million at December 31, 2019, were measured at fair value using net asset value per share (or its practical expedient) and are not 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.