Segment Information - Service and Fee Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue from External Customer
Service and fee income	$ 184,969	$ 161,626	$ 545,002	$ 476,041
Reciprocal Exchanges
Revenue from External Customer
Service and fee income	1,628	1,585	5,121	4,471
NGHC, excluding Reciprocal Exchanges
Revenue from External Customer
Service and fee income	183,341	160,041	539,881	471,570
Property and Casualty
Revenue from External Customer
Service and fee income	96,372	97,890	295,813	298,356
Property and Casualty | Reciprocal Exchanges
Revenue from External Customer
Service and fee income	1,628	1,585	5,121	4,471
Property and Casualty | NGHC, excluding Reciprocal Exchanges
Revenue from External Customer
Service and fee income	94,744	96,305	290,692	293,885
Accident and Health
Revenue from External Customer
Service and fee income	88,597	63,736	249,189	177,685
Accident and Health | Reciprocal Exchanges
Revenue from External Customer
Service and fee income	0	0	0	0
Accident and Health | NGHC, excluding Reciprocal Exchanges
Revenue from External Customer
Service and fee income	88,597	63,736	249,189	177,685
Commission revenue
Revenue from External Customer
Service and fee income	33,635	37,686	124,288	125,290
Commission revenue | Property and Casualty
Revenue from External Customer
Service and fee income	11,799	21,333	48,466	68,193
Commission revenue | Accident and Health
Revenue from External Customer
Service and fee income	21,836	16,353	75,822	57,097
Finance and processing fees
Revenue from External Customer
Service and fee income	34,783	34,437	103,434	101,928
Finance and processing fees | Property and Casualty
Revenue from External Customer
Service and fee income	32,237	33,637	96,212	98,218
Finance and processing fees | Accident and Health
Revenue from External Customer
Service and fee income	2,546	800	7,222	3,710
Group health administrative fees
Revenue from External Customer
Service and fee income	31,147	25,808	91,658	73,861
Group health administrative fees | Property and Casualty
Revenue from External Customer
Service and fee income	0	0	0	0
Group health administrative fees | Accident and Health
Revenue from External Customer
Service and fee income	31,147	25,808	91,658	73,861
Installment fees
Revenue from External Customer
Service and fee income	26,789	23,464	78,282	72,782
Installment fees | Property and Casualty
Revenue from External Customer
Service and fee income	26,789	23,464	78,282	72,782
Installment fees | Accident and Health
Revenue from External Customer
Service and fee income	0	0	0	0
Late payment fees
Revenue from External Customer
Service and fee income	8,340	9,323	23,946	26,310
Late payment fees | Property and Casualty
Revenue from External Customer
Service and fee income	8,324	9,251	23,905	26,061
Late payment fees | Accident and Health
Revenue from External Customer
Service and fee income	16	72	41	249
Other service and fee income
Revenue from External Customer
Service and fee income	50,275	30,908	123,394	75,870
Other service and fee income | Property and Casualty
Revenue from External Customer
Service and fee income	17,223	10,205	48,948	33,102
Other service and fee income | Accident and Health
Revenue from External Customer
Service and fee income	$ 33,052	$ 20,703	$ 74,446	$ 42,768