Schedule I - Summary of Investments-Other than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 1,434,701
Amount at which shown in the balance sheet	1,540,392
Debt Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,253,166
Fair value	1,275,016
Amount at which shown in the balance sheet	1,275,016
US Treasury Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	8,115
Fair value	8,078
Amount at which shown in the balance sheet	8,078
US Government-sponsored Enterprises Debt Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	303,932
Fair value	297,949
Amount at which shown in the balance sheet	297,949
Obligations of States and Political Subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	290,038
Fair value	307,536
Amount at which shown in the balance sheet	307,536
Commercial Mortgage Backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	84,058
Fair value	83,980
Amount at which shown in the balance sheet	83,980
Residential Mortgage Backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	120,554
Fair value	119,799
Amount at which shown in the balance sheet	119,799
Other Asset Backed [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	23,934
Fair value	24,114
Amount at which shown in the balance sheet	24,114
Corporate Debt Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	422,535
Fair value	433,560
Amount at which shown in the balance sheet	433,560
Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	144,274
Fair value	228,115
Amount at which shown in the balance sheet	228,115
Common stocks Financial services [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	30,103
Fair value	43,522
Amount at which shown in the balance sheet	43,522
Common stocks Information technology [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	18,308
Fair value	35,810
Amount at which shown in the balance sheet	35,810
Common stocks Healthcare [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	18,877
Fair value	30,595
Amount at which shown in the balance sheet	30,595
Common stocks Consumer staples [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	9,275
Fair value	14,127
Amount at which shown in the balance sheet	14,127
Common stocks Consumer discretionary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	10,935
Fair value	20,538
Amount at which shown in the balance sheet	20,538
Common stocks Energy [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	12,441
Fair value	16,905
Amount at which shown in the balance sheet	16,905
Common stocks Industrials [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	12,746
Fair value	28,489
Amount at which shown in the balance sheet	28,489
Common stocks Other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	11,058
Fair value	16,421
Amount at which shown in the balance sheet	16,421
Nonredeemable Preferred Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	20,531
Fair value	21,708
Amount at which shown in the balance sheet	21,708
Other Long-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	13,648
Amount at which shown in the balance sheet	13,648
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	23,613
Fair value	23,613
Amount at which shown in the balance sheet	$ 23,613