Segment Information (Schedule of revenue by insurance product line) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Direct premiums earned										$ 301,061	$ 257,661	$ 219,600
Assumed premiums earned										6,459	5,897	6,514
Ceded premiums earned										(23,859)	(17,120)	(30,394)
Net premiums earned		$ 69,541	$ 73,342	$ 82,006	$ 58,772	$ 63,702	$ 67,116	$ 65,114	$ 50,506	283,661	246,438	195,720
Direct losses and LAE										185,370	173,943	119,894
Assumed losses and LAE										3,308	4,032	3,399
Ceded losses and LAE										(20,205)	(8,265)	(4,205)
Net losses and LAE										168,473	169,710	119,088
Gross margin										115,188	76,728	76,632
Underwriting and general expenses										85,068	67,258	54,117
Underwriting gain (loss)										30,120	9,470	22,515
Fee and other income										1,801	2,125	6,496
Net investment income		1,396	1,886	2,018	1,971	1,929	1,983	1,778	1,743	7,271	7,433	6,180
Net capital gain on investments										13,624	14,783	3,974
Income before income taxes										52,816	33,811	39,165
Income taxes										11,472	7,311	7,921
Net income		22,446	3,686	18,767	(3,555)	17,148	(6,959)	2,515	13,796	41,344	26,500	31,244
Net income attributable to non-controlling interest										955	99	163
Net income attributable to NI Holdings, Inc.		21,579	$ 3,664	$ 18,733	$ (3,587)	17,129	$ (6,979)	$ 2,478	$ 13,773	$ 40,389	$ 26,401	$ 31,081
Non-GAAP Ratios:
Loss and LAE ratio										59.40%	68.90%	60.80%
Expense ratio										30.00%	27.30%	27.70%
Combined ratio										89.40%	96.20%	88.50%
Premiums and agents' balances receivable		48,523				36,691				$ 48,523	$ 36,691	$ 34,287
Deferred policy acquisition costs		23,968				15,399				23,968	15,399	12,866	$ 8,859
Reinsurance recoverables		8,710				4,045				8,710	4,045	2,232
Receivable from Federal Crop Insurance Corporation		6,646				14,230				6,646	14,230	16,169
Goodwill and other intangibles		9,384				2,628				9,384	2,628	4,623
Unpaid losses and LAE		105,750				93,250				105,750	93,250	87,121
Unearned premiums		119,363				89,276				119,363	89,276	84,767
Gain on acquisition of Direct Auto Insurance Company												4,578
Private Passenger Auto [Member]
Direct premiums earned										74,998	71,297	65,457
Assumed premiums earned												16
Ceded premiums earned										(2,989)	(3,314)	(3,008)
Net premiums earned										72,009	67,983	62,465
Direct losses and LAE										45,423	53,022	44,644
Assumed losses and LAE											63	358
Ceded losses and LAE											(389)	(478)
Ceded losses and LAE										88
Net losses and LAE										45,511	52,696	44,524
Gross margin										26,498	15,287	17,941
Underwriting and general expenses										19,986	18,886	18,530
Underwriting gain (loss)										$ 6,512	$ (3,599)	$ (589)
Non-GAAP Ratios:
Loss and LAE ratio										63.20%	77.50%	71.30%
Expense ratio										27.80%	27.80%	29.70%
Combined ratio										91.00%	105.30%	100.90%
Premiums and agents' balances receivable		18,540				18,194				$ 18,540	$ 18,194	$ 16,558
Deferred policy acquisition costs		5,461				4,108				5,461	4,108	3,658
Reinsurance recoverables		412				500				412	500	176
Receivable from Federal Crop Insurance Corporation
Goodwill and other intangibles
Unpaid losses and LAE		20,311				19,892				20,311	19,892	18,154
Unearned premiums		28,293				27,949				28,293	27,949	26,022
Non-standard auto (Primero) [Member]
Direct premiums earned										53,909	57,278	27,964
Assumed premiums earned
Ceded premiums earned										(172)	(164)
Net premiums earned										53,737	57,114	27,964
Direct losses and LAE										30,347	32,654	14,338
Assumed losses and LAE
Ceded losses and LAE
Net losses and LAE										30,347	32,654	14,338
Gross margin										23,390	24,460	13,626
Underwriting and general expenses										20,739	21,077	9,533
Underwriting gain (loss)										2,651	3,383	4,093
Fee and other income										1,337	1,638	1,406
Other income net										$ 3,988	$ 5,021	$ 5,499
Non-GAAP Ratios:
Loss and LAE ratio										56.50%	57.20%	51.30%
Expense ratio										38.60%	36.90%	34.10%
Combined ratio										95.10%	94.10%	85.40%
Premiums and agents' balances receivable		6,543				7,876				$ 6,543	$ 7,876	$ 7,769
Deferred policy acquisition costs		4,649				4,711				4,649	4,711	3,364
Reinsurance recoverables
Receivable from Federal Crop Insurance Corporation
Goodwill and other intangibles		2,860				2,912				2,860	2,912	4,623
Unpaid losses and LAE		43,336				43,978				43,336	43,978	46,786
Unearned premiums		16,147				16,364				16,147	16,364	17,177
Percentage effect on net premiums earned	6.00%
Home and Farm [Member]
Direct premiums earned										82,036	77,832	70,355
Assumed premiums earned
Assumed premiums earned												(17)
Ceded premiums earned										(7,157)	(6,661)	(5,661)
Net premiums earned										74,879	71,171	64,677
Direct losses and LAE										38,700	47,282	42,993
Assumed losses and LAE
Assumed losses and LAE										(116)		(295)
Ceded losses and LAE										(1,839)	(1,681)
Ceded losses and LAE												17
Net losses and LAE										36,745	45,601	42,715
Gross margin										38,134	25,570	21,962
Underwriting and general expenses										20,874	20,106	19,950
Underwriting gain (loss)										$ 17,260	$ 5,464	$ 2,012
Non-GAAP Ratios:
Loss and LAE ratio										49.10%	64.10%	66.00%
Expense ratio										27.90%	28.30%	30.80%
Combined ratio										76.90%	92.30%	96.90%
Premiums and agents' balances receivable		9,072				9,088				$ 9,072	$ 9,088	$ 8,630
Deferred policy acquisition costs		7,828				5,945				7,828	5,945	5,279
Reinsurance recoverables		588				1,068				588	1,068	730
Receivable from Federal Crop Insurance Corporation
Goodwill and other intangibles
Unpaid losses and LAE		11,737				10,503				11,737	10,503	10,732
Unearned premiums		41,301				39,945				41,301	39,945	36,883
Crop [Member]
Direct premiums earned										39,893	42,277	47,505
Assumed premiums earned										1,896	2,072	2,306
Ceded premiums earned										(6,071)	(6,330)	(21,112)
Net premiums earned										35,718	38,019	28,699
Direct losses and LAE										36,022	35,148	14,206
Assumed losses and LAE										1,070	1,582	696
Ceded losses and LAE										(5,713)	(4,639)	(2,990)
Net losses and LAE										31,379	32,091	11,912
Gross margin										4,339	5,928	16,787
Underwriting and general expenses										4,807	4,396	3,176
Underwriting gain (loss)										$ (468)	$ 1,532	$ 13,611
Non-GAAP Ratios:
Loss and LAE ratio										87.90%	84.40%	41.50%
Expense ratio										13.50%	11.60%	11.10%
Combined ratio										101.30%	96.00%	52.60%
Premiums and agents' balances receivable
Deferred policy acquisition costs
Reinsurance recoverables		121				998				121	998	384
Receivable from Federal Crop Insurance Corporation		6,646				14,230				6,646	14,230	16,169
Goodwill and other intangibles
Unpaid losses and LAE		771				8,579				771	8,579	2,126
Unearned premiums
Commercial [Member]
Direct premiums earned										45,557	4,546	4,230
Assumed premiums earned											1	1
Ceded premiums earned										(7,269)	(450)	(422)
Net premiums earned										38,288	4,097	3,809
Direct losses and LAE										32,620	2,541	1,690
Assumed losses and LAE
Ceded losses and LAE										(12,190)	(52)	(186)
Net losses and LAE										20,430	2,489
Gross margin										17,858	1,608	2,305
Underwriting and general expenses										16,358	863	875
Underwriting gain (loss)										$ 1,500	$ 745	$ 1,430
Non-GAAP Ratios:
Loss and LAE ratio										53.40%	60.80%	39.50%
Expense ratio										42.70%	21.10%	23.00%
Combined ratio										96.10%	81.80%	62.50%
Premiums and agents' balances receivable		13,732				940				$ 13,732	$ 940	$ 854
Deferred policy acquisition costs		5,588				315				5,588	315	280
Reinsurance recoverables		5,374				6				5,374	6
Reinsurance recoverables												(28)
Receivable from Federal Crop Insurance Corporation
Goodwill and other intangibles		15,334								15,334
Unpaid losses and LAE		19,089				1,076				19,089	1,076	643
Unearned premiums		30,705				2,334				30,705	2,334	2,200
All Other [Member]
Direct premiums earned										4,668	4,431	4,089
Assumed premiums earned										4,563	3,824	4,208
Ceded premiums earned										(201)	(201)	(191)
Net premiums earned										9,030	8,054	8,106
Direct losses and LAE										2,258	3,296	2,023
Assumed losses and LAE										2,354	2,387	2,640
Ceded losses and LAE										(551)	(1,504)	(568)
Net losses and LAE										4,061	4,179	4,095
Gross margin										4,969	3,875	4,011
Underwriting and general expenses										2,304	1,930	2,053
Underwriting gain (loss)										$ 2,665	$ 1,945	$ 1,958
Non-GAAP Ratios:
Loss and LAE ratio										45.00%	51.90%	50.50%
Expense ratio										25.50%	24.00%	25.30%
Combined ratio										70.50%	75.90%	75.80%
Premiums and agents' balances receivable		636				593				$ 636	$ 593	$ 476
Deferred policy acquisition costs		442				320				442	320	285
Reinsurance recoverables		2,215				1,473				2,215	1,473	970
Receivable from Federal Crop Insurance Corporation
Goodwill and other intangibles
Unpaid losses and LAE		10,506				9,222				10,506	9,222	8,680
Unearned premiums		$ 2,917				$ 2,684				$ 2,917	$ 2,684	$ 2,485