Schedule I - Summary of Investments - Other Than Investments In Related Parties (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	$ 3,801,356
Fair Value	3,934,062
Amount Which is Presented in the Balance Sheet	3,925,696
U.S. Government or government agencies and authorities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	176,224
Fair Value	176,774
Amount Which is Presented in the Balance Sheet	176,774
State and municipal bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	790,154
Fair Value	800,463
Amount Which is Presented in the Balance Sheet	800,463
Foreign governments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	5,488
Fair Value	5,524
Amount Which is Presented in the Balance Sheet	5,524
Public utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	88,886
Fair Value	91,038
Amount Which is Presented in the Balance Sheet	91,038
All other corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	1,170,288
Fair Value	1,182,279
Amount Which is Presented in the Balance Sheet	1,182,279
Certificates of deposit
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	150
Fair Value	150
Amount Which is Presented in the Balance Sheet	150
Mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	355,631
Fair Value	357,178
Amount Which is Presented in the Balance Sheet	357,178
Total Fixed Maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	2,586,821
Fair Value	2,613,406
Amount Which is Presented in the Balance Sheet	2,613,406
Public utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	10,365
Fair Value	12,100
Amount Which is Presented in the Balance Sheet	12,100
Banks, trusts and insurance companies
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	72,416
Fair Value	81,749
Amount Which is Presented in the Balance Sheet	81,749
Industrial, miscellaneous and all other
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	270,963
Fair Value	293,425
Amount Which is Presented in the Balance Sheet	293,425
Total Equity Securities, trading
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	353,744
Fair Value	387,274
Amount Which is Presented in the Balance Sheet	387,274
Other long-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	418,707
Fair Value	491,298
Amount Which is Presented in the Balance Sheet	482,932
Short-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Recorded Cost Basis	442,084
Fair Value	442,084
Amount Which is Presented in the Balance Sheet	$ 442,084