Segment Information - Results of Operations of the Business Segments (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)
Segment Reporting [Abstract]
Number of business segments | segment			2
Segment Reporting Information
Gross premium written	$ 1,473,127	$ 1,435,157	$ 4,291,282	$ 4,258,843
Ceded premiums	(207,871)	(400,553)	(801,515)	(1,064,177)
Net premium written	1,265,256	1,034,604	3,489,767	3,194,666
Change in unearned premium	(86,683)	19,034	(170,421)	(146,220)
Net earned premium	1,178,573	1,053,638	3,319,346	3,048,446
Ceding commission income	30,724	57,587	127,669	187,313
Service and fee income	184,969	161,626	545,002	476,041
Total underwriting revenues	1,394,266	1,272,851	3,992,017	3,711,800
Loss and loss adjustment expense	774,164	745,334	2,066,608	2,112,678
Acquisition costs and other underwriting expenses	268,645	209,090	726,265	615,134
General and administrative expenses	244,611	263,864	774,589	759,725
Total underwriting expenses	1,287,420	1,218,288	3,567,462	3,487,537
Underwriting income	106,846	54,563	424,555	224,263
Net investment income	28,904	33,740	90,322	102,316
Net gain (loss) on investments	19,839	1,581	18,136	(3,627)
Interest expense	(11,292)	(12,898)	(34,851)	(38,822)
Provision for income taxes	(27,428)	(16,747)	(106,107)	(61,494)
Net (income) loss attributable to noncontrolling interest	(6,900)	10,915	(14,753)	18,152
Net income attributable to NGHC	109,969	71,154	377,302	240,788
Property and Casualty
Segment Reporting Information
Gross premium written	1,277,190	1,272,602	3,718,663	3,666,072
Ceded premiums	(186,264)	(385,347)	(739,654)	(971,643)
Net premium written	1,090,926	887,255	2,979,009	2,694,429
Change in unearned premium	(87,757)	(2,520)	(168,570)	(142,515)
Net earned premium	1,003,169	884,735	2,810,439	2,551,914
Ceding commission income	30,519	55,326	126,433	178,533
Service and fee income	96,372	97,890	295,813	298,356
Total underwriting revenues	1,130,060	1,037,951	3,232,685	3,028,803
Loss and loss adjustment expense	696,216	674,722	1,840,404	1,870,993
Acquisition costs and other underwriting expenses	189,367	161,876	511,622	462,071
General and administrative expenses	182,146	195,567	594,381	571,498
Total underwriting expenses	1,067,729	1,032,165	2,946,407	2,904,562
Underwriting income	62,331	5,786	286,278	124,241
Net investment income	0	0	0	0
Net gain (loss) on investments	0	0	0	0
Interest expense	0	0	0	0
Provision for income taxes	0	0	0	0
Net (income) loss attributable to noncontrolling interest	0	0	0	0
Net income attributable to NGHC	62,331	5,786	286,278	124,241
Accident and Health
Segment Reporting Information
Gross premium written	195,937	162,555	572,619	592,771
Ceded premiums	(21,607)	(15,206)	(61,861)	(92,534)
Net premium written	174,330	147,349	510,758	500,237
Change in unearned premium	1,074	21,554	(1,851)	(3,705)
Net earned premium	175,404	168,903	508,907	496,532
Ceding commission income	205	2,261	1,236	8,780
Service and fee income	88,597	63,736	249,189	177,685
Total underwriting revenues	264,206	234,900	759,332	682,997
Loss and loss adjustment expense	77,948	70,612	226,204	241,685
Acquisition costs and other underwriting expenses	79,278	47,214	214,643	153,063
General and administrative expenses	58,892	68,297	176,635	188,227
Total underwriting expenses	216,118	186,123	617,482	582,975
Underwriting income	48,088	48,777	141,850	100,022
Net investment income	0	0	0	0
Net gain (loss) on investments	0	0	0	0
Interest expense	0	0	0	0
Provision for income taxes	0	0	0	0
Net (income) loss attributable to noncontrolling interest	0	0	0	0
Net income attributable to NGHC	48,088	48,777	141,850	100,022
Corporate and Other
Segment Reporting Information
Gross premium written	0	0	0	0
Ceded premiums	0	0	0	0
Net premium written	0	0	0	0
Change in unearned premium	0	0	0	0
Net earned premium	0	0	0	0
Ceding commission income	0	0	0	0
Service and fee income	0	0	0	0
Total underwriting revenues	0	0	0	0
Loss and loss adjustment expense	0	0	0	0
Acquisition costs and other underwriting expenses	0	0	0	0
General and administrative expenses	3,573	0	3,573	0
Total underwriting expenses	3,573	0	3,573	0
Underwriting income	(3,573)	0	(3,573)	0
Net investment income	28,904	33,740	90,322	102,316
Net gain (loss) on investments	19,839	1,581	18,136	(3,627)
Interest expense	(11,292)	(12,898)	(34,851)	(38,822)
Provision for income taxes	(27,428)	(16,747)	(106,107)	(61,494)
Net (income) loss attributable to noncontrolling interest	(6,900)	10,915	(14,753)	18,152
Net income attributable to NGHC	$ (450)	$ 16,591	$ (50,826)	$ 16,525