Segment Information (Schedule of revenue by insurance product line) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Direct premiums earned	$ 53,987	$ 48,564	$ 91,179	$ 82,680
Assumed premiums earned	3,249	3,368	4,488	4,539
Ceded premiums earned	(6,559)	(6,279)	(8,878)	(8,757)
Net premiums earned	50,677	45,653	86,789	78,462
Direct losses and LAE	41,121	36,093	59,758	55,735
Assumed losses and LAE	487	2,024	1,076	2,613
Ceded losses and LAE	(887)	(1,887)	(1,264)	(4,397)
Net losses and LAE	40,721	36,230	59,570	53,951
Gross margin	9,956	9,423	27,219	24,511
Underwriting and general expenses	11,802	11,425	23,679	21,444
Underwriting gain (loss)	(1,846)	(2,002)	3,540	3,067
Fee and other income	470	321	847	668
Net investment income	1,523	1,303	2,892	2,302
Net realized capital gain on investments	250	339	719	955
Income loss before income taxes	397	(39)	7,998	6,992
Income taxes (benefit)	141	(156)	1,590	2,126
Net income	256	117	6,408	4,866
Net income loss attributable to non-controlling interest	30	(12)	60	48
Net income attributable to NI Holdings, Inc.	$ 226	$ 129	$ 6,348	$ 4,818
Loss and LAE ratio	80.40%	79.40%	68.60%	68.80%
Expense ratio	23.30%	25.00%	27.30%	27.30%
Combined ratio	103.60%	104.40%	95.90%	96.10%
Premiums receivable	$ 74,175	$ 70,882	$ 74,175	$ 70,882		$ 25,632
Deferred policy acquisition costs	12,190	14,126	12,190	14,126	$ 8,980	8,859	$ 9,681	$ 8,942
Reinsurance recoverables	3,601	10,392	3,601	10,392		4,128
Receivable from Federal Crop Insurance Corporation	11,130	12,496	11,130	12,496		10,501
Unpaid losses and LAE	63,376	72,619	63,376	72,619		45,890
Unearned premiums	99,954	92,194	99,954	92,194		$ 63,262
Private Passenger Auto [Member]
Direct premiums earned	16,139	14,278	31,718	27,877
Assumed premiums earned	4	4	8	8
Ceded premiums earned	(833)	(762)	(1,591)	(1,721)
Net premiums earned	15,310	13,520	30,135	26,164
Direct losses and LAE	11,177	7,432	21,364	17,461
Assumed losses and LAE	47	39	47	310
Ceded losses and LAE	(49)	262	(114)	(51)
Net losses and LAE	11,175	7,733	21,297	17,720
Gross margin	4,135	5,787	8,838	8,444
Underwriting and general expenses	4,703	5,921	9,122	10,128
Underwriting gain (loss)	$ (568)	$ (134)	$ (284)	$ (1,684)
Loss and LAE ratio	73.00%	57.20%	70.70%	67.70%
Expense ratio	30.70%	43.80%	30.30%	38.70%
Combined ratio	103.70%	101.00%	100.90%	106.40%
Premiums receivable	$ 17,365	$ 15,600	$ 17,365	$ 15,600
Deferred policy acquisition costs	3,754	2,577	3,754	2,577
Reinsurance recoverables	432	1,435	432	1,435
Receivable from Federal Crop Insurance Corporation
Unpaid losses and LAE	18,592	24,391	18,592	24,391
Unearned premiums	27,248	25,247	27,248	25,247
Non Standard Auto [Member]
Direct premiums earned	3,476	2,590	6,467	5,121
Assumed premiums earned
Ceded premiums earned
Net premiums earned	3,476	2,590	6,467	5,121
Direct losses and LAE	2,973	2,347	4,937	4,719
Assumed losses and LAE
Ceded losses and LAE
Net losses and LAE	2,973	2,347	4,937	4,719
Gross margin	503	243	1,530	402
Underwriting and general expenses	890	807	1,925	1,492
Underwriting gain (loss)	(387)	(564)	(395)	(1,090)
Fee and other income	330	262	651	526
Other income net	$ (57)	$ (302)	$ 256	$ (564)
Loss and LAE ratio	85.50%	90.60%	76.30%	92.10%
Expense ratio	25.60%	31.20%	29.80%	29.10%
Combined ratio	111.10%	121.80%	106.10%	121.30%
Premiums receivable	$ 1,126	$ 1,003	$ 1,126	$ 1,003
Deferred policy acquisition costs	322	277	322	277
Reinsurance recoverables
Receivable from Federal Crop Insurance Corporation
Unpaid losses and LAE	5,942	6,276	5,942	6,276
Unearned premiums	1,860	1,579	1,860	1,579
Home and Farm [Member]
Direct premiums earned	17,243	15,649	33,862	30,721
Assumed premiums earned	(4)	1	(8)	2
Ceded premiums earned	(1,618)	(1,579)	(3,115)	(3,014)
Net premiums earned	15,621	14,071	30,739	27,709
Direct losses and LAE	16,542	14,224	21,958	19,509
Assumed losses and LAE	(62)		(49)
Ceded losses and LAE	47	347	(60)	(506)
Net losses and LAE	16,527	14,571	21,849	19,003
Gross margin	(906)	(500)	8,890	8,706
Underwriting and general expenses	5,138	6,264	10,092	9,777
Underwriting gain (loss)	$ (6,044)	$ (6,764)	$ (1,202)	$ (1,071)
Loss and LAE ratio	105.80%	103.60%	71.10%	68.60%
Expense ratio	32.90%	44.50%	32.80%	35.30%
Combined ratio	138.70%	148.10%	103.90%	103.90%
Premiums receivable	$ 9,819	$ 8,328	$ 9,819	$ 8,328
Deferred policy acquisition costs	5,250	5,238	5,250	5,238
Reinsurance recoverables	589	2,625	589	2,625
Receivable from Federal Crop Insurance Corporation
Unpaid losses and LAE	18,838	17,162	18,838	17,162
Unearned premiums	37,473	33,894	37,473	33,894
Crop [Member]
Direct premiums earned	15,084	14,138	15,084	15,164
Assumed premiums earned	2,283	2,435	2,283	2,437
Ceded premiums earned	(3,926)	(3,759)	(3,825)	(3,687)
Net premiums earned	13,441	12,814	13,542	13,914
Direct losses and LAE	9,724	11,573	10,063	12,106
Assumed losses and LAE	164	1,403	164	1,403
Ceded losses and LAE	(881)	(2,062)	(905)	(2,015)
Net losses and LAE	9,007	10,914	9,322	11,494
Gross margin	4,434	1,900	4,220	2,420
Underwriting and general expenses	163	(2,244)	651	(1,392)
Underwriting gain (loss)	$ 4,271	$ 4,144	$ 3,569	$ 3,812
Loss and LAE ratio	67.00%	85.20%	68.80%	82.60%
Expense ratio	1.20%	(17.50%)	4.80%	(10.00%)
Combined ratio	68.20%	67.70%	73.60%	72.60%
Premiums receivable	$ 44,397	$ 44,736	$ 44,397	$ 44,736
Deferred policy acquisition costs	2,267	5,586	2,267	5,586
Reinsurance recoverables	899	3,716	899	3,716
Receivable from Federal Crop Insurance Corporation	11,130	12,496	11,130	12,496
Unpaid losses and LAE	10,020	15,862	10,020	15,862
Unearned premiums	28,310	26,718	28,310	26,718
All Other [Member]
Direct premiums earned	2,045	1,909	4,048	3,797
Assumed premiums earned	966	928	2,205	2,092
Ceded premiums earned	(182)	(179)	(347)	(335)
Net premiums earned	2,829	2,658	5,906	5,554
Direct losses and LAE	705	517	1,436	1,940
Assumed losses and LAE	338	582	914	900
Ceded losses and LAE	(4)	(434)	(185)	(1,825)
Net losses and LAE	1,039	665	2,165	1,015
Gross margin	1,790	1,993	3,741	4,539
Underwriting and general expenses	908	677	1,889	1,439
Underwriting gain (loss)	$ 882	$ 1,316	$ 1,852	$ 3,100
Loss and LAE ratio	36.70%	25.00%	36.70%	18.30%
Expense ratio	32.10%	25.50%	32.00%	25.90%
Combined ratio	68.80%	50.50%	68.60%	44.20%
Premiums receivable	$ 1,468	$ 1,215	$ 1,468	$ 1,215
Deferred policy acquisition costs	597	448	597	448
Reinsurance recoverables	1,681	2,616	1,681	2,616
Receivable from Federal Crop Insurance Corporation
Unpaid losses and LAE	9,984	8,928	9,984	8,928
Unearned premiums	$ 5,063	$ 4,756	$ 5,063	$ 4,756