Schedule I - Summary of Investments - Other Than Investments In Related Parties (Details) $ in Thousands	Dec. 31, 2019 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	$ 3,199,182
Fair Value	3,390,409
Amount Which is Presented in the Balance Sheet	3,390,409
U.S. Government or government agencies and authorities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	138,708
Fair Value	140,336
Amount Which is Presented in the Balance Sheet	140,336
State and municipal bonds
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	287,658
Fair Value	296,093
Amount Which is Presented in the Balance Sheet	296,093
Foreign governments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	39,208
Fair Value	40,124
Amount Which is Presented in the Balance Sheet	40,124
Public utilities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	75,294
Fair Value	77,342
Amount Which is Presented in the Balance Sheet	77,342
All other corporate bonds
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	1,227,444
Fair Value	1,256,376
Amount Which is Presented in the Balance Sheet	1,256,376
Asset-backed securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	519,764
Fair Value	525,798
Amount Which is Presented in the Balance Sheet	525,798
Total Fixed Maturities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	2,288,076
Fair Value	2,336,069
Amount Which is Presented in the Balance Sheet	2,336,069
Public utilities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	3,567
Fair Value	4,407
Amount Which is Presented in the Balance Sheet	4,407
Banks, trusts and insurance companies
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	35,548
Fair Value	40,294
Amount Which is Presented in the Balance Sheet	40,294
Industrial, miscellaneous and all other
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	188,758
Fair Value	205,851
Amount Which is Presented in the Balance Sheet	205,851
Equity investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	227,873
Fair Value	250,552
Amount Which is Presented in the Balance Sheet	250,552
Other long-term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	343,334
Fair Value	463,881
Amount Which is Presented in the Balance Sheet	463,881
Short-term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Recorded Cost Basis	339,899
Fair Value	339,907
Amount Which is Presented in the Balance Sheet	$ 339,907