Schedule I - Summary of Investments-Other than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2015 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 1,322,922
Fair value	1,415,797
Amount at which shown in the balance sheet	1,415,797
Debt Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,130,217
Fair value	1,161,025
Amount at which shown in the balance sheet	1,161,025
US Treasury Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	12,566
Fair value	12,589
Amount at which shown in the balance sheet	12,589
US Government-sponsored Enterprises Debt Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	202,486
Fair value	202,666
Amount at which shown in the balance sheet	202,666
Obligations of States and Political Subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	319,940
Fair value	344,359
Amount at which shown in the balance sheet	344,359
Commercial Mortgage Backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	44,433
Fair value	46,108
Amount at which shown in the balance sheet	46,108
Residential Mortgage Backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	94,279
Fair value	88,543
Amount at which shown in the balance sheet	88,543
Other Asset Backed [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	17,000
Fair value	17,844
Amount at which shown in the balance sheet	17,844
Corporate Debt Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	439,513
Fair value	448,916
Amount at which shown in the balance sheet	448,916
Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	144,176
Fair value	206,243
Amount at which shown in the balance sheet	206,243
Common stocks Financial services [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	24,557
Fair value	33,955
Amount at which shown in the balance sheet	33,955
Common stocks Information technology [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	19,427
Fair value	28,102
Amount at which shown in the balance sheet	28,102
Common stocks Healthcare [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	15,599
Fair value	25,894
Amount at which shown in the balance sheet	25,894
Common stocks Consumer staples [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	11,136
Fair value	18,200
Amount at which shown in the balance sheet	18,200
Common stocks Consumer discretionary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	10,270
Fair value	18,923
Amount at which shown in the balance sheet	18,923
Common stocks Energy [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	16,384
Fair value	21,068
Amount at which shown in the balance sheet	21,068
Common stocks Industrials [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	11,525
Fair value	20,416
Amount at which shown in the balance sheet	20,416
Common stocks Other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	17,246
Fair value	20,683
Amount at which shown in the balance sheet	20,683
Nonredeemable Preferred Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	18,032
Fair value	19,002
Amount at which shown in the balance sheet	19,002
Other Long-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	9,930
Fair value	9,930
Amount at which shown in the balance sheet	9,930
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	38,599
Fair value	38,599
Amount at which shown in the balance sheet	$ 38,599