Segment Information - Net Earned Premium (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information
Net earned premium	$ 1,178,573	$ 1,053,638	$ 3,319,346	$ 3,048,446
NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net earned premium	1,124,475	996,521	3,152,865	2,899,041
Property and Casualty
Segment Reporting Information
Net earned premium	1,003,169	884,735	2,810,439	2,551,914
Property and Casualty | Reciprocal Exchanges
Segment Reporting Information
Net earned premium	54,098	57,117	166,481	149,405
Property and Casualty | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net earned premium	949,071	827,618	2,643,958	2,402,509
Property and Casualty | Personal Auto | Reciprocal Exchanges
Segment Reporting Information
Net earned premium	24,654	33,953	89,291	65,907
Property and Casualty | Personal Auto | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net earned premium	632,781	568,346	1,773,831	1,621,734
Property and Casualty | Homeowners | Reciprocal Exchanges
Segment Reporting Information
Net earned premium	28,963	22,759	76,037	82,475
Property and Casualty | Homeowners | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net earned premium	130,278	86,525	321,129	272,591
Property and Casualty | RV/Packaged | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net earned premium	52,126	51,023	148,374	150,739
Property and Casualty | Small Business Auto | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net earned premium	53,158	62,265	166,316	189,957
Property and Casualty | Lender-placed insurance | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net earned premium	77,441	56,599	218,210	158,595
Property and Casualty | Other | Reciprocal Exchanges
Segment Reporting Information
Net earned premium	481	405	1,153	1,023
Property and Casualty | Other | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net earned premium	3,287	2,860	16,098	8,893
Accident and Health
Segment Reporting Information
Net earned premium	175,404	168,903	508,907	496,532
Accident and Health | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net earned premium	175,404	168,903	508,907	496,532
Accident and Health | Group | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net earned premium	70,670	59,009	209,372	170,921
Accident and Health | Individual | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net earned premium	104,734	85,971	299,535	252,122
Accident and Health | International | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net earned premium	$ 0	$ 23,923	$ 0	$ 73,489