Investments (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) Director Security Investment	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) Security	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Director Security Partnership Investment	Dec. 31, 2016 USD ($) Security	Dec. 31, 2015 USD ($)
Net Investment Income [Line Items]
Investment Income									$ 20,422	$ 16,980	$ 15,140
Investment expenses									(2,327)	(2,497)	(2,642)
Net investment income									18,095	14,483	12,498
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Fair value	$ 723,616				$ 611,849				723,616	611,849
Cost or amortized cost	651,768				559,463				651,768	559,463
Gross unrealized gains	78,904				60,621				78,904	60,621
Gross unrealized losses	(7,056)				(8,235)				(7,056)	(8,235)
Net unrealized gains (losses)	$ 71,848				$ 52,386				71,848	52,386
Applicable federal income taxes									(25,148)	(18,335)
Net unrealized gains - net of tax									$ 46,700	$ 34,051
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Number of securities, total | Security	636				486				636	486
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, total	$ 435,713				$ 343,800				$ 435,713	$ 343,800
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, total	(7,056)				(8,235)				(7,056)	(8,235)
Gain and Loss on Investment [Abstract]
Limited partnerships - net gain (loss)									12,469	2,473	(1,687)
Total net gains (losses)	4,152	$ 5,944	$ 3,296	$ 6,294	6,204	$ 7,732	$ 280	$ 9,012	19,686	23,228	(1,261)
Adjustments for other-than-temporary impairment [Roll Forward]
Cumulative charges to income at beginning of year				$ 5,060				$ 10,513	5,060	10,513	7,168
Writedowns based on objective and subjective criteria									149	5,743	7,700
Recovery of prior writedowns upon sale or disposal									(1,590)	(10,606)	(4,355)
Net pre-tax realized gain (loss)									1,441	4,863	(3,345)
Cumulative charges to income at end of year	4,209				5,060				4,209	5,060	10,513
Limited Partners' Capital Account [Line Items]
Market value of equity and fixed maturity securities portfolio managed by broker dealers	24,779								24,779
Total commissions and net fees earned by broker dealers and affiliates									97	207	235
Summarized financial information of significant limited partnership investments [Abstract]
Total assets	1,357,016				1,154,137				1,357,016	1,154,137
Total liabilities	938,205				749,792				938,205	749,792
Total partners' capital	43,586				44,038				43,586	44,038
Regulatory deposits with various insurance departments in United States and Canada, fair value	86,335				86,169				86,335	86,169
Time certificate of deposit included in short-term investments	1,000								$ 1,000
Percentage of fixed maturities invested in investment grade fixed maturity									86.00%
Total fixed maturity investments	$ 693								$ 693
Number of different investments in fixed maturity | Investment	1								1
Fixed maturity investment below investment grade	$ 71,907								$ 71,907
Percentage of fixed maturity investment to total invested assets									8.40%
Number of investments in diversified portfolio including catastrophe bonds | Investment	40								40
Net unrealized loss position of diversified portfolio									$ 1,605
Directors [Member]
Limited Partners' Capital Account [Line Items]
Number of company employees employed by investment firm | Director	2								2
Limited Partnership [Member]
Limited Partners' Capital Account [Line Items]
Commitment to make additional contributions to various limited partnerships	$ 1,404								$ 1,404
Number of investments of the partnership | Partnership									3
Aggregate estimated value in limited partnership	43,586								$ 43,586
Number of directors that are executive officers, directors, and owners of the organization managing the limited partnership | Director									2
Investment gains (losses), net of fees	$ 9,549				(971)				$ 9,549	(971)	(1,978)
Management fees									$ 803	777	749
Number of significant investments in public and private equity market in India | Investment	1								1
Significant investment in public and private equity markets in India, value	$ 29,817				27,153				$ 29,817	27,153
Estimated ownership interest in significant limited partnership investment									5.00%
Earnings from limited partnership investment									$ 7,665	(1,117)	(1,599)
Summarized financial information of significant limited partnership investments [Abstract]
Investment income (loss)									623	(5)	(188)
Partnership expenses									2,206	2,426	3,024
Net investment loss									(1,583)	(2,431)	(3,212)
Realized gain (loss) on investments									8,723	7,754	21,091
Unrealized appreciation (depreciation) on investments	133,807				(21,002)				133,807	(21,002)	(37,482)
Net increase (decrease) in partners' capital resulting from operations									140,947	(15,679)	(19,603)
Total assets	566,629				448,263				566,629	448,263	511,118
Total liabilities	30,976				39,988				30,976	39,988	40,335
Total partners' capital	$ 535,653				408,275				$ 535,653	408,275	470,783
Limited Partnership [Member] | New Vernon India Fund [Member]
Limited Partners' Capital Account [Line Items]
Ownership interest in limited partnership	5.00%								5.00%
Withdrew from fund									$ 5,000
Limited Partnership [Member] | New Vernon Global Opportunity Fund [Member]
Limited Partners' Capital Account [Line Items]
Ownership interest in limited partnership	37.00%								37.00%
Limited Partnership [Member] | New Vernon Global Opportunity Fund II [Member]
Limited Partners' Capital Account [Line Items]
Ownership interest in limited partnership	23.00%								23.00%
Withdrew from fund									$ 5,000
Agency Collateralized Mortgage Obligations [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value	$ 16,586				6,171				16,586	6,171
Cost or amortized cost	15,839				6,000				15,839	6,000
Gross unrealized gains	818				171				818	171
Gross unrealized losses	(71)				0				(71)	0
Net unrealized gains (losses)	747				171				747	171
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	16,586				6,171				16,586	6,171
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Cost or amortized cost	15,839				6,000				15,839	6,000
Agency Mortgage Backed Securities [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value	27,075				4,770				27,075	4,770
Cost or amortized cost	27,180				4,751				27,180	4,751
Gross unrealized gains	47				57				47	57
Gross unrealized losses	(152)				(38)				(152)	(38)
Net unrealized gains (losses)	(105)				19				(105)	19
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	27,075				4,770				27,075	4,770
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Cost or amortized cost	27,180				4,751				27,180	4,751
Asset-backed Securities [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value	43,469				45,183				43,469	45,183
Cost or amortized cost	42,861				45,207				42,861	45,207
Gross unrealized gains	749				458				749	458
Gross unrealized losses	(141)				(482)				(141)	(482)
Net unrealized gains (losses)	608				(24)				608	(24)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	43,469				45,183				43,469	45,183
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Cost or amortized cost	42,861				45,207				42,861	45,207
Bank Loans [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value	19,488				10,349				19,488	10,349
Cost or amortized cost	19,271				10,222				19,271	10,222
Gross unrealized gains	266				149				266	149
Gross unrealized losses	(49)				(22)				(49)	(22)
Net unrealized gains (losses)	217				127				217	127
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	19,488				10,349				19,488	10,349
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Cost or amortized cost	19,271				10,222				19,271	10,222
Certificates of Deposit [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value	3,135				3,117				3,135	3,117
Cost or amortized cost	3,124				3,126				3,124	3,126
Gross unrealized gains	11				0				11	0
Gross unrealized losses	0				(9)				0	(9)
Net unrealized gains (losses)	11				(9)				11	(9)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	3,135				3,117				3,135	3,117
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Cost or amortized cost	3,124				3,126				3,124	3,126
Collateralized Mortgage Obligations [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value	6,492				9,104				6,492	9,104
Cost or amortized cost	6,079				9,096				6,079	9,096
Gross unrealized gains	451				290				451	290
Gross unrealized losses	(38)				(282)				(38)	(282)
Net unrealized gains (losses)	413				8				413	8
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	6,492				9,104				6,492	9,104
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Cost or amortized cost	6,079				9,096				6,079	9,096
Corporate Securities [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value	198,349				142,683				198,349	142,683
Cost or amortized cost	198,419				143,356				198,419	143,356
Gross unrealized gains	1,602				1,643				1,602	1,643
Gross unrealized losses	(1,672)				(2,316)				(1,672)	(2,316)
Net unrealized gains (losses)	(70)				(673)				(70)	(673)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	198,349				142,683				198,349	142,683
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Cost or amortized cost	198,419				143,356				198,419	143,356
Mortgage-backed Securities [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value	24,204				24,571				24,204	24,571
Cost or amortized cost	23,656				23,904				23,656	23,904
Gross unrealized gains	933				1,132				933	1,132
Gross unrealized losses	(385)				(465)				(385)	(465)
Net unrealized gains (losses)	548				667				548	667
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	24,204				24,571				24,204	24,571
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Cost or amortized cost	23,656				23,904				23,656	23,904
Municipal Obligations [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value	96,650				129,335				96,650	129,335
Cost or amortized cost	97,059				130,204				97,059	130,204
Gross unrealized gains	322				391				322	391
Gross unrealized losses	(731)				(1,260)				(731)	(1,260)
Net unrealized gains (losses)	(409)				(869)				(409)	(869)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	96,650				129,335				96,650	129,335
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Cost or amortized cost	97,059				130,204				97,059	130,204
Non-U.S. Government Obligations [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value	37,394				24,681				37,394	24,681
Cost or amortized cost	37,971				26,461				37,971	26,461
Gross unrealized gains	475				230				475	230
Gross unrealized losses	(1,052)				(2,010)				(1,052)	(2,010)
Net unrealized gains (losses)	(577)				(1,780)				(577)	(1,780)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	37,394				24,681				37,394	24,681
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Cost or amortized cost	37,971				26,461				37,971	26,461
U.S. Government Obligations [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value	49,011				91,940				49,011	91,940
Cost or amortized cost	49,558				92,234				49,558	92,234
Gross unrealized gains	0				74				0	74
Gross unrealized losses	(547)				(368)				(547)	(368)
Net unrealized gains (losses)	(547)				(294)				(547)	(294)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	49,011				91,940				49,011	91,940
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Cost or amortized cost	49,558				92,234				49,558	92,234
Total Fixed Maturities [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value	521,853				491,904				521,853	491,904
Cost or amortized cost	521,017				494,561				521,017	494,561
Gross unrealized gains	5,674				4,595				5,674	4,595
Gross unrealized losses	(4,838)				(7,252)				(4,838)	(7,252)
Net unrealized gains (losses)	$ 836				$ (2,657)				$ 836	$ (2,657)
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Number of Securities 12 months or less | Security	459				397				459	397
Number of securities, Greater than 12 months | Security	112				54				112	54
Number of securities, total | Security	571				451				571	451
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less	$ 313,421				$ 291,048				$ 313,421	$ 291,048
Fair value, Greater than 12 months	75,638				32,054				75,638	32,054
Fair value, total	389,059				323,102				389,059	323,102
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, less than 12 months	(2,683)				(4,380)				(2,683)	(4,380)
Gross unrealized loss, 12 months or longer	(2,155)				(2,872)				(2,155)	(2,872)
Gross unrealized loss, total	(4,838)				(7,252)				(4,838)	(7,252)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
One year or less, Fair Value	54,567								54,567
Excess of one year to five years, Fair Value	292,755								292,755
Excess of five years to ten years, Fair Value	61,087								61,087
Excess of ten years, Fair Value	2,909								2,909
Total contractual maturities, Fair Value	411,318								411,318
Asset-backed securities, Fair Value	110,535								110,535
Total, Fair Value	$ 521,853				491,904				$ 521,853	491,904
Percentage of Available for sale Securities, Debt Maturities, Fair Value [Abstract]
One year or less, Fair Value	10.50%								10.50%
Excess of one year to five years, Fair Value	56.10%								56.10%
Excess of five years to ten years, Fair Value	11.70%								11.70%
Excess of ten years, Fair Value	0.50%								0.50%
Total maturities, Fair Value	78.80%								78.80%
Asset-backed securities, Fair Value	21.20%								21.20%
Total, Fair Value	100.00%								100.00%
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
One year or less, Cost or Amortized Cost	$ 54,777								$ 54,777
Excess of one year to five years, Cost or Amortized Cost	294,698								294,698
Excess of five years to ten years, Cost or Amortized Cost	60,366								60,366
Excess of ten years, Cost or Amortized Cost	2,914								2,914
Total contractual maturities, Cost or Amortized Cost	412,755								412,755
Asset-backed securities, Cost or Amortized Cost	108,262								108,262
Cost or amortized cost	$ 521,017				494,561				$ 521,017	494,561
Percentage of Available for sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
One year or less, Cost or Amortized Cost	10.50%								10.50%
Excess of one year to five years, Cost or Amortized Cost	56.60%								56.60%
Excess of five years to ten years, Cost or Amortized Cost	11.60%								11.60%
Excess of ten years, Cost or Amortized Cost	0.50%								0.50%
Total maturities, Cost or Amortized Cost	79.20%								79.20%
Asset-backed securities, Cost or Amortized Cost	20.80%								20.80%
Cost or amortized cost	100.00%								100.00%
Gain and Loss on Investment [Abstract]
Gross gains									$ 9,135	11,628	6,633
Gross losses									(10,031)	(14,020)	(13,634)
Net losses									(896)	(2,392)	(7,001)
Consumer [Member]
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Fair value	$ 46,578				32,576				46,578	32,576
Cost or amortized cost	23,565				15,231				23,565	15,231
Gross unrealized gains	24,031				17,656				24,031	17,656
Gross unrealized losses	(1,018)				(311)				(1,018)	(311)
Net unrealized gains (losses)	23,013				17,345				23,013	17,345
Energy [Member]
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Fair value	10,278				12,842				10,278	12,842
Cost or amortized cost	6,763				5,641				6,763	5,641
Gross unrealized gains	3,602				7,203				3,602	7,203
Gross unrealized losses	(87)				(2)				(87)	(2)
Net unrealized gains (losses)	3,515				7,201				3,515	7,201
Financial [Member]
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Fair value	45,470				31,186				45,470	31,186
Cost or amortized cost	31,859				22,417				31,859	22,417
Gross unrealized gains	13,937				8,998				13,937	8,998
Gross unrealized losses	(326)				(229)				(326)	(229)
Net unrealized gains (losses)	13,611				8,769				13,611	8,769
Industrial [Member]
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Fair value	25,402				21,145				25,402	21,145
Cost or amortized cost	8,949				6,239				8,949	6,239
Gross unrealized gains	16,793				15,098				16,793	15,098
Gross unrealized losses	(340)				(192)				(340)	(192)
Net unrealized gains (losses)	16,453				14,906				16,453	14,906
Technology [Member]
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Fair value	13,061				8,858				13,061	8,858
Cost or amortized cost	5,768				4,117				5,768	4,117
Gross unrealized gains	7,401				4,769				7,401	4,769
Gross unrealized losses	(108)				(28)				(108)	(28)
Net unrealized gains (losses)	7,293				4,741				7,293	4,741
Funds (e.g. mutual funds, closed end funds, ETFs) [Member]
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Fair value	50,291				6,995				50,291	6,995
Cost or amortized cost	46,177				6,930				46,177	6,930
Gross unrealized gains	4,153				121				4,153	121
Gross unrealized losses	(39)				(56)				(39)	(56)
Net unrealized gains (losses)	4,114				65				4,114	65
Other [Member]
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Fair value	10,683				6,343				10,683	6,343
Cost or amortized cost	7,670				4,327				7,670	4,327
Gross unrealized gains	3,313				2,181				3,313	2,181
Gross unrealized losses	(300)				(165)				(300)	(165)
Net unrealized gains (losses)	3,013				2,016				3,013	2,016
Total Equity Securities [Member]
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Fair value	201,763				119,945				201,763	119,945
Cost or amortized cost	130,751				64,902				130,751	64,902
Gross unrealized gains	73,230				56,026				73,230	56,026
Gross unrealized losses	(2,218)				(983)				(2,218)	(983)
Net unrealized gains (losses)	$ 71,012				$ 55,043				$ 71,012	$ 55,043
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Number of Securities 12 months or less | Security	65				35				65	35
Number of securities, Greater than 12 months | Security	0				0				0	0
Number of securities, total | Security	65				35				65	35
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less	$ 46,654				$ 20,698				$ 46,654	$ 20,698
Fair value, Greater than 12 months	0				0				0	0
Fair value, total	46,654				20,698				46,654	20,698
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, less than 12 months	(2,218)				(983)				(2,218)	(983)
Gross unrealized loss, 12 months or longer	0				0				0	0
Gross unrealized loss, total	$ (2,218)				$ (983)				(2,218)	(983)
Gain and Loss on Investment [Abstract]
Gross gains									10,481	28,742	21,070
Gross losses									(2,368)	(5,595)	(13,643)
Net losses									8,113	23,147	7,427
Total Fixed Maturities [Member]
Net Investment Income [Line Items]
Investment Income									15,340	13,254	11,663
Dividends on Equity Securities [Member]
Net Investment Income [Line Items]
Investment Income									4,611	3,598	3,445
Money Market Funds, Short term and Other [Member]
Net Investment Income [Line Items]
Investment Income									$ 471	$ 128	$ 32