Schedule I - Summary of Investments Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2020 USD ($)
Summary of Investments, Other than Investments in Related Parties [Abstract]
Investments classified as cash and cash equivalents	$ 47,026
Total Investments [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	954,507
Fair value	990,286
Amount at which shown in the consolidated balance sheet	989,463	[1]
Fixed Income Securities [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	894,468
Fair value	919,692
Amount at which shown in the consolidated balance sheet	919,692	[1]
Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	48,437
Fair value	58,169
Amount at which shown in the consolidated balance sheet	58,169	[1]
Bonds [Member] | Agency Collateralized Mortgage Obligations [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	11,448
Fair value	11,931
Amount at which shown in the consolidated balance sheet	11,931	[1]
Bonds [Member] | Agency Mortgage-backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	99,060
Fair value	102,107
Amount at which shown in the consolidated balance sheet	102,107	[1]
Bonds [Member] | Asset-backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	108,686
Fair value	107,696
Amount at which shown in the consolidated balance sheet	107,696	[1]
Bonds [Member] | Bank Loans [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	11,590
Fair value	11,361
Amount at which shown in the consolidated balance sheet	11,361	[1]
Bonds [Member] | Collateralized Mortgage Obligations [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	5,061
Fair value	5,118
Amount at which shown in the consolidated balance sheet	5,118	[1]
Bonds [Member] | Corporate Securities [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	344,059
Fair value	360,241
Amount at which shown in the consolidated balance sheet	360,241	[1]
Bonds [Member] | Mortgage-backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	40,675
Fair value	38,056
Amount at which shown in the consolidated balance sheet	38,056	[1]
Bonds [Member] | Municipal Obligations [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	43,353
Fair value	45,143
Amount at which shown in the consolidated balance sheet	45,143	[1]
Bonds [Member] | Non-U.S. Government Obligations [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	29,882
Fair value	30,600
Amount at which shown in the consolidated balance sheet	30,600	[1]
Bonds [Member] | U.S. Government Obligations [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	200,654
Fair value	207,439
Amount at which shown in the consolidated balance sheet	207,439	[1]
Common Stocks [Member] | Consumer [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	9,619
Fair value	11,598
Amount at which shown in the consolidated balance sheet	11,598	[1]
Common Stocks [Member] | Energy [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	2,046
Fair value	1,227
Amount at which shown in the consolidated balance sheet	1,227	[1]
Common Stocks [Member] | Financial [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	24,007
Fair value	29,064
Amount at which shown in the consolidated balance sheet	29,064	[1]
Common Stocks [Member] | Industrial [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	4,066
Fair value	5,180
Amount at which shown in the consolidated balance sheet	5,180	[1]
Common Stocks [Member] | Technology [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	1,749
Fair value	2,851
Amount at which shown in the consolidated balance sheet	2,851	[1]
Common Stocks [Member] | Other [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	6,950
Fair value	8,249
Amount at which shown in the consolidated balance sheet	8,249	[1]
Commercial Mortgage Loans [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	10,602
Fair value	11,425
Amount at which shown in the consolidated balance sheet	10,602	[1]
Total Short-term and Other [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	1,000
Fair value	1,000
Amount at which shown in the consolidated balance sheet	1,000	[1]
Short-term [Member] | Certificates of Deposit [Member]
Summary of Investments, Other than Investments in Related Parties [Abstract]
Cost	1,000
Fair value	1,000
Amount at which shown in the consolidated balance sheet	$ 1,000	[1]

[1] Amounts presented above do not include investments of $47,026 classified as cash and cash equivalents in the consolidated balance sheet.