Schedule I - Summary of Investments-Other than Investments in Related Parties (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 1,263,467
Fair value	1,384,024
Amount at which shown in the balance sheet	1,384,024
Debt Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,080,006
Fair value	1,127,499
Amount at which shown in the balance sheet	1,127,499
US Treasury Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	9,574
Fair value	9,703
Amount at which shown in the balance sheet	9,703
US Government-sponsored Enterprises Debt Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	215,425
Fair value	215,616
Amount at which shown in the balance sheet	215,616
Obligations of States and Political Subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	299,258
Fair value	326,058
Amount at which shown in the balance sheet	326,058
Commercial Mortgage Backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	42,996
Fair value	46,762
Amount at which shown in the balance sheet	46,762
Residential Mortgage Backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	100,296
Fair value	97,953
Amount at which shown in the balance sheet	97,953
Other Asset Backed [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	14,798
Fair value	16,005
Amount at which shown in the balance sheet	16,005
Corporate Debt Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	397,659
Fair value	415,402
Amount at which shown in the balance sheet	415,402
Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	123,972
Fair value	197,036
Amount at which shown in the balance sheet	197,036
Common stocks Financial services [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	22,586
Fair value	34,379
Amount at which shown in the balance sheet	34,379
Common stocks Information technology [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	15,755
Fair value	26,865
Amount at which shown in the balance sheet	26,865
Common stocks Healthcare [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	14,673
Fair value	26,852
Amount at which shown in the balance sheet	26,852
Common stocks Consumer staples [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	10,584
Fair value	16,694
Amount at which shown in the balance sheet	16,694
Common stocks Consumer discretionary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	11,304
Fair value	22,691
Amount at which shown in the balance sheet	22,691
Common stocks Energy [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	15,837
Fair value	22,863
Amount at which shown in the balance sheet	22,863
Common stocks Industrials [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	9,658
Fair value	18,221
Amount at which shown in the balance sheet	18,221
Common stocks Other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	11,493
Fair value	16,056
Amount at which shown in the balance sheet	16,056
Nonredeemable Preferred Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	12,082
Fair value	12,415
Amount at which shown in the balance sheet	12,415
Other Long-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6,227
Fair value	6,227
Amount at which shown in the balance sheet	6,227
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	53,262
Fair value	53,262
Amount at which shown in the balance sheet	$ 53,262