Segment Information - Results of Operations of the Business Segments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)
Segment Reporting [Abstract]
Number of business segments | segment	2
Segment Reporting Information
Gross premium written	$ 1,476,554	$ 1,509,778
Ceded premiums	(266,951)	(345,114)
Net premium written	1,209,603	1,164,664
Change in unearned premium	(134,397)	(200,507)
Net earned premium	1,075,206	964,157
Ceding commission income	50,305	69,534
Service and fee income	179,441	165,507
Total underwriting revenues	1,304,952	1,199,198
Loss and loss adjustment expense	691,998	651,809
Acquisition costs and other underwriting expenses	228,242	211,918
General and administrative expenses	267,569	248,094
Total underwriting expenses	1,187,809	1,111,821
Underwriting income	117,143	87,377
Net investment income	30,243	33,445
Net gain (loss) on Investments	(6,861)	22
Interest expense	(11,780)	(12,999)
Provision for income taxes	(28,172)	(22,506)
Net loss attributable to noncontrolling interest	186	6,419
Net income attributable to NGHC	100,759	91,758
Property and Casualty
Segment Reporting Information
Gross premium written	1,289,529	1,251,234
Ceded premiums	(248,142)	(286,751)
Net premium written	1,041,387	964,483
Change in unearned premium	(130,887)	(161,906)
Net earned premium	910,500	802,577
Ceding commission income	49,745	66,943
Service and fee income	98,917	104,495
Total underwriting revenues	1,059,162	974,015
Loss and loss adjustment expense	610,397	567,060
Acquisition costs and other underwriting expenses	159,771	154,070
General and administrative expenses	207,817	189,456
Total underwriting expenses	977,985	910,586
Underwriting income	81,177	63,429
Net investment income	0	0
Net gain (loss) on Investments	0	0
Interest expense	0	0
Provision for income taxes	0	0
Net loss attributable to noncontrolling interest	0	0
Net income attributable to NGHC	81,177	63,429
Accident and Health
Segment Reporting Information
Gross premium written	187,025	258,544
Ceded premiums	(18,809)	(58,363)
Net premium written	168,216	200,181
Change in unearned premium	(3,510)	(38,601)
Net earned premium	164,706	161,580
Ceding commission income	560	2,591
Service and fee income	80,524	61,012
Total underwriting revenues	245,790	225,183
Loss and loss adjustment expense	81,601	84,749
Acquisition costs and other underwriting expenses	68,471	57,848
General and administrative expenses	59,752	58,638
Total underwriting expenses	209,824	201,235
Underwriting income	35,966	23,948
Net investment income	0	0
Net gain (loss) on Investments	0	0
Interest expense	0	0
Provision for income taxes	0	0
Net loss attributable to noncontrolling interest	0	0
Net income attributable to NGHC	35,966	23,948
Corporate and Other
Segment Reporting Information
Gross premium written	0	0
Ceded premiums	0	0
Net premium written	0	0
Change in unearned premium	0	0
Net earned premium	0	0
Ceding commission income	0	0
Service and fee income	0	0
Total underwriting revenues	0	0
Loss and loss adjustment expense	0	0
Acquisition costs and other underwriting expenses	0	0
General and administrative expenses	0	0
Total underwriting expenses	0	0
Underwriting income	0	0
Net investment income	30,243	33,445
Net gain (loss) on Investments	(6,861)	22
Interest expense	(11,780)	(12,999)
Provision for income taxes	(28,172)	(22,506)
Net loss attributable to noncontrolling interest	186	6,419
Net income attributable to NGHC	$ (16,384)	$ 4,381