Schedule I - Summary of Investments-Other than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	$ 1,482,223
Amount at which shown in the balance sheet	1,546,992
Equity investments, at fair value	215,363
Debt Securities [Member]
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,273,132
Fair value	1,282,909
Amount at which shown in the balance sheet	1,282,909
U.S. treasury
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	8,139
Fair value	8,021
Amount at which shown in the balance sheet	8,021
U.S. government-sponsored agencies
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	303,198
Fair value	304,479
Amount at which shown in the balance sheet	304,479
Obligations of States and Political Subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	273,727
Fair value	283,651
Amount at which shown in the balance sheet	283,651
Commercial mortgage-backed
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	83,854
Fair value	84,379
Amount at which shown in the balance sheet	84,379
Residential mortgage-backed
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	161,055
Fair value	162,137
Amount at which shown in the balance sheet	162,137
Other Asset Backed [Member]
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	21,596
Fair value	20,834
Amount at which shown in the balance sheet	20,834
Corporate
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	421,563
Fair value	419,408
Amount at which shown in the balance sheet	419,408
Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	160,371
Fair value	215,363
Amount at which shown in the balance sheet	215,363
Financial services
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	35,410
Fair value	41,839
Amount at which shown in the balance sheet	41,839
Information technology
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	17,678
Fair value	31,581
Amount at which shown in the balance sheet	31,581
Healthcare
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	21,602
Fair value	34,571
Amount at which shown in the balance sheet	34,571
Consumer staples
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	10,926
Fair value	13,180
Amount at which shown in the balance sheet	13,180
Consumer discretionary
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	15,921
Fair value	22,765
Amount at which shown in the balance sheet	22,765
Energy
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	10,124
Fair value	13,372
Amount at which shown in the balance sheet	13,372
Industrials
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	10,989
Fair value	19,389
Amount at which shown in the balance sheet	19,389
Other
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	11,512
Fair value	14,371
Amount at which shown in the balance sheet	14,371
Non-redeemable preferred stocks
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	18,531
Fair value	16,654
Amount at which shown in the balance sheet	16,654
Private Equity Funds, US [Member]
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	7,678
Fair value	7,641
Amount at which shown in the balance sheet	7,641
Investment funds
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,200
Amount at which shown in the balance sheet	1,200
Other Long-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	19,316
Amount at which shown in the balance sheet	19,316
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	28,204
Fair value	28,204
Amount at which shown in the balance sheet	$ 28,204