Segment Information - Net Premium Written (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 premium written	$ 1,265,256	$ 1,034,604	$ 3,489,767	$ 3,194,666
NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net premium written	1,235,765	967,319	3,347,713	3,022,206
Property and Casualty
Segment Reporting Information
Net premium written	1,090,926	887,255	2,979,009	2,694,429
Property and Casualty | Reciprocal Exchanges
Segment Reporting Information
Net premium written	29,491	67,285	142,054	172,460
Property and Casualty | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net premium written	1,061,435	819,970	2,836,955	2,521,969
Property and Casualty | Personal Auto | Reciprocal Exchanges
Segment Reporting Information
Net premium written	10,323	67,154	73,678	101,460
Property and Casualty | Personal Auto | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net premium written	650,889	560,032	1,886,438	1,730,904
Property and Casualty | Homeowners | Reciprocal Exchanges
Segment Reporting Information
Net premium written	18,166	(2,047)	66,758	68,180
Property and Casualty | Homeowners | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net premium written	217,960	82,601	390,760	276,250
Property and Casualty | RV/Packaged | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net premium written	57,834	52,283	165,694	162,047
Property and Casualty | Small Business Auto | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net premium written	47,556	56,615	152,584	196,221
Property and Casualty | Lender-placed insurance | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net premium written	82,599	61,579	224,742	140,863
Property and Casualty | Other | Reciprocal Exchanges
Segment Reporting Information
Net premium written	1,002	2,178	1,618	2,820
Property and Casualty | Other | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net premium written	4,597	6,860	16,737	15,684
Accident and Health
Segment Reporting Information
Net premium written	174,330	147,349	510,758	500,237
Accident and Health | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net premium written	174,330	147,349	510,758	500,237
Accident and Health | Group | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net premium written	70,672	59,001	209,360	170,911
Accident and Health | Individual | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net premium written	103,658	85,541	301,398	252,316
Accident and Health | International | NGHC, excluding Reciprocal Exchanges
Segment Reporting Information
Net premium written	$ 0	$ 2,807	$ 0	$ 77,010