Reserves for unpaid losses and loss adjustment expenses Reserve Development (Details)	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)
Claims Development
Prior year losses and loss adjustment expense	$ (9,423,000)	$ (7,037,000)	$ (11,280,000)
Liability for claims and claim adjustment expense, net	356,108,000
Property [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	37,234,000
All outstanding liabilities before 2015, net of reinsurance	0
Liability for claims and claim adjustment expense, net	5,507,000
Incurred claims and claim adjustment expense, net	42,741,000
Casualty - claims made [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	57,785,000
Liability for claims and claim adjustment expense, net	64,929,000
Incurred claims and claim adjustment expense, net	122,714,000
Casualty - occurrence [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	192,981,000
Liability for claims and claim adjustment expense, net	285,672,000
Incurred claims and claim adjustment expense, net	478,653,000
Accident Year 2010 [Member] | Casualty - claims made [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	859,000	859,000	859,000	$ 859,000	$ 859,000	$ 862,000	$ 393,000	$ 368,000	$ 79,000	$ 0
Incurred claims and claim adjustment expense, net	872,000	876,000	883,000	894,000	916,000	946,000	737,000	679,000	805,000	778,000
Total of IBNR liabilities plus expected development on reported claims	$ 13,000
Cumulative number of reported claims	14
Accident Year 2010 [Member] | Casualty - occurrence [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 347,000	346,000	313,000	273,000	261,000	263,000	113,000	59,000	37,000	4,000
Incurred claims and claim adjustment expense, net	406,000	402,000	429,000	406,000	404,000	458,000	460,000	531,000	771,000	$ 843,000
Total of IBNR liabilities plus expected development on reported claims	$ 59,000
Cumulative number of reported claims	49
Accident Year 2011 [Member] | Casualty - claims made [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 3,104,000	3,065,000	3,042,000	3,042,000	3,044,000	2,116,000	1,392,000	1,037,000	139,000
Incurred claims and claim adjustment expense, net	3,206,000	3,225,000	3,261,000	3,374,000	3,560,000	3,609,000	3,646,000	3,844,000	4,246,000
Total of IBNR liabilities plus expected development on reported claims	$ 102,000
Cumulative number of reported claims	76
Accident Year 2011 [Member] | Casualty - occurrence [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 8,693,000	8,510,000	6,868,000	6,363,000	4,575,000	3,788,000	2,519,000	1,596,000	207,000
Incurred claims and claim adjustment expense, net	9,379,000	9,023,000	8,375,000	8,142,000	7,613,000	7,340,000	5,757,000	5,940,000	$ 5,839,000
Total of IBNR liabilities plus expected development on reported claims	$ 471,000
Cumulative number of reported claims	226
Accident Year 2012 [Member] | Casualty - claims made [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 1,946,000	1,946,000	1,882,000	1,090,000	1,024,000	877,000	475,000	153,000
Incurred claims and claim adjustment expense, net	2,207,000	2,314,000	2,477,000	2,845,000	3,102,000	4,205,000	5,749,000	7,913,000
Total of IBNR liabilities plus expected development on reported claims	$ 261,000
Cumulative number of reported claims	138
Accident Year 2012 [Member] | Casualty - occurrence [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 20,707,000	19,383,000	16,474,000	14,382,000	11,238,000	7,850,000	4,441,000	757,000
Incurred claims and claim adjustment expense, net	23,223,000	22,401,000	22,001,000	20,579,000	20,401,000	18,803,000	17,436,000	$ 16,977,000
Total of IBNR liabilities plus expected development on reported claims	$ 1,716,000
Cumulative number of reported claims	581
Accident Year 2013 [Member] | Casualty - claims made [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 5,482,000	5,439,000	5,320,000	5,070,000	4,436,000	1,915,000	499,000
Incurred claims and claim adjustment expense, net	6,128,000	6,463,000	7,002,000	7,917,000	9,113,000	11,639,000	15,238,000
Total of IBNR liabilities plus expected development on reported claims	$ 629,000
Cumulative number of reported claims	228
Accident Year 2013 [Member] | Casualty - occurrence [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 28,248,000	26,715,000	21,348,000	14,890,000	7,957,000	4,469,000	1,099,000
Incurred claims and claim adjustment expense, net	33,973,000	33,248,000	31,731,000	29,039,000	28,037,000	28,771,000	$ 30,616,000
Total of IBNR liabilities plus expected development on reported claims	$ 4,016,000
Cumulative number of reported claims	840
Accident Year 2014 [Member] | Casualty - claims made [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 9,415,000	8,290,000	6,384,000	5,039,000	1,865,000	435,000
Incurred claims and claim adjustment expense, net	10,988,000	10,948,000	11,217,000	11,748,000	14,289,000	18,847,000
Total of IBNR liabilities plus expected development on reported claims	$ 1,239,000
Cumulative number of reported claims	273
Accident Year 2014 [Member] | Casualty - occurrence [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 31,026,000	23,771,000	17,576,000	8,489,000	3,081,000	698,000
Incurred claims and claim adjustment expense, net	41,859,000	39,313,000	36,678,000	38,049,000	40,668,000	$ 47,805,000
Total of IBNR liabilities plus expected development on reported claims	$ 7,980,000
Cumulative number of reported claims	1,158
Accident Year 2015 [Member] | Property [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 869,000	865,000	832,000	706,000	584,000
Incurred claims and claim adjustment expense, net	869,000	872,000	857,000	880,000	1,394,000
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of reported claims	63
Accident Year 2015 [Member] | Casualty - claims made [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 11,371,000	9,238,000	7,562,000	4,496,000	217,000
Incurred claims and claim adjustment expense, net	13,942,000	13,583,000	14,896,000	16,777,000	18,883,000
Total of IBNR liabilities plus expected development on reported claims	$ 2,024,000
Cumulative number of reported claims	257
Accident Year 2015 [Member] | Casualty - occurrence [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 37,690,000	28,385,000	12,685,000	3,161,000	941,000
Incurred claims and claim adjustment expense, net	54,697,000	52,100,000	49,122,000	51,739,000	$ 59,717,000
Total of IBNR liabilities plus expected development on reported claims	$ 13,117,000
Cumulative number of reported claims	1,322
Accident Year 2016 [Member]
Claims Development
Prior year losses and loss adjustment expense		(3,800,000)	(9,400,000)
Accident Year 2016 [Member] | Property [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 3,265,000	3,265,000	3,257,000	1,867,000
Incurred claims and claim adjustment expense, net	3,278,000	3,301,000	3,392,000	4,177,000
Total of IBNR liabilities plus expected development on reported claims	$ 14,000
Cumulative number of reported claims	286
Accident Year 2016 [Member] | Casualty - claims made [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 9,839,000	6,907,000	3,015,000	1,158,000
Incurred claims and claim adjustment expense, net	14,322,000	14,675,000	14,694,000	19,170,000
Total of IBNR liabilities plus expected development on reported claims	$ 3,248,000
Cumulative number of reported claims	309
Accident Year 2016 [Member] | Casualty - occurrence [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 28,924,000	17,225,000	6,015,000	1,099,000
Incurred claims and claim adjustment expense, net	55,534,000	53,549,000	55,680,000	$ 61,440,000
Total of IBNR liabilities plus expected development on reported claims	$ 19,588,000
Cumulative number of reported claims	1,318
Accident Year 2017 [Member]
Claims Development
Prior year losses and loss adjustment expense	$ (1,600,000)	(6,800,000)
Accident Year 2017 [Member] | Property [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	11,602,000	11,233,000	9,938,000
Incurred claims and claim adjustment expense, net	11,676,000	11,705,000	12,473,000
Total of IBNR liabilities plus expected development on reported claims	$ 65,000
Cumulative number of reported claims	1,038
Accident Year 2017 [Member] | Casualty - claims made [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 8,252,000	4,897,000	340,000
Incurred claims and claim adjustment expense, net	16,121,000	17,096,000	18,116,000
Total of IBNR liabilities plus expected development on reported claims	$ 5,024,000
Cumulative number of reported claims	366
Accident Year 2017 [Member] | Casualty - occurrence [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 22,407,000	9,352,000	1,581,000
Incurred claims and claim adjustment expense, net	68,985,000	67,151,000	$ 71,126,000
Total of IBNR liabilities plus expected development on reported claims	$ 33,798,000
Cumulative number of reported claims	1,373
Accident Years 2011 to 2015 [Member]
Claims Development
Prior year losses and loss adjustment expense	$ 5,200,000	3,600,000
Accident Year 2018 [Member]
Claims Development
Prior year losses and loss adjustment expense	(13,000,000.0)
Accident Year 2018 [Member] | Property [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	11,646,000	9,132,000
Incurred claims and claim adjustment expense, net	12,004,000	11,559,000
Total of IBNR liabilities plus expected development on reported claims	$ 236,000
Cumulative number of reported claims	643
Accident Year 2018 [Member] | Casualty - claims made [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 5,030,000	507,000
Incurred claims and claim adjustment expense, net	20,235,000	22,429,000
Total of IBNR liabilities plus expected development on reported claims	$ 10,949,000
Cumulative number of reported claims	448
Accident Year 2018 [Member] | Casualty - occurrence [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 10,995,000	2,638,000
Incurred claims and claim adjustment expense, net	78,331,000	$ 86,157,000
Total of IBNR liabilities plus expected development on reported claims	$ 56,169,000
Cumulative number of reported claims	1,297
Accident Year 2019 [Member] | Property [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 9,852,000
Incurred claims and claim adjustment expense, net	14,914,000
Total of IBNR liabilities plus expected development on reported claims	$ 2,384,000
Cumulative number of reported claims	598
Accident Year 2019 [Member] | Casualty - claims made [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 2,487,000
Incurred claims and claim adjustment expense, net	34,693,000
Total of IBNR liabilities plus expected development on reported claims	$ 27,450,000
Cumulative number of reported claims	504
Accident Year 2019 [Member] | Casualty - occurrence [Member]
Claims Development
Cumulative paid claims and allocated claim adjustment expense, net	$ 3,944,000
Incurred claims and claim adjustment expense, net	112,266,000
Total of IBNR liabilities plus expected development on reported claims	$ 99,970,000
Cumulative number of reported claims	951