Investments - Gross Unrealized Losses (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020 USD ($) security	Sep. 30, 2020 USD ($) security	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) security
Schedule of Available-for-sale Securities
Less Than 12 Months	$ 420,077	$ 420,077		$ 1,187,942
Less Than 12 Months	(4,975)	(4,975)		(7,993)
12 Months or More	53,683	53,683		124,587
12 Months or More	(2,182)	(2,182)		(2,770)
Total	473,760	473,760		1,312,529
Total	$ (7,157)	$ (7,157)		$ (10,763)
Number of securities with gross unrealized loss | security	276	276		1,337
25% of amortized cost or cost	25.00%	25.00%		25.00%
Debt securities, available-for sale, allowance for expected credit losses	$ 1,085	$ 1,085	$ 1,203	$ 0
Debt Securities, Available-for-sale, Change in Present Value, Expense (Reversal)	(100)	(1,824)
Reciprocal Exchanges
Schedule of Available-for-sale Securities
Less Than 12 Months	64,479	64,479		83,698
Less Than 12 Months	(306)	(306)		(339)
12 Months or More	3,912	3,912		6,906
12 Months or More	(88)	(88)		(81)
Total	68,391	68,391		90,604
Total	(394)	(394)		(420)
Debt securities, available-for sale, allowance for expected credit losses	150	150
NGHC, excluding Reciprocal Exchanges
Schedule of Available-for-sale Securities
Less Than 12 Months	355,598	355,598		1,104,244
Less Than 12 Months	(4,669)	(4,669)		(7,654)
12 Months or More	49,771	49,771		117,681
12 Months or More	(2,094)	(2,094)		(2,689)
Total	405,369	405,369		1,221,925
Total	(6,763)	(6,763)		(10,343)
Debt securities, available-for sale, allowance for expected credit losses	935	935
U.S. Treasury
Schedule of Available-for-sale Securities
Less Than 12 Months				19,903
Less Than 12 Months				(23)
12 Months or More				500
12 Months or More				0
Total				20,403
Total				(23)
Debt securities, available-for sale, allowance for expected credit losses	0	0
States and political subdivision bonds
Schedule of Available-for-sale Securities
Less Than 12 Months	16,935	16,935		106,103
Less Than 12 Months	(92)	(92)		(1,415)
12 Months or More	0	0		2,580
12 Months or More	0	0		(26)
Total	16,935	16,935		108,683
Total	(92)	(92)		(1,441)
Debt securities, available-for sale, allowance for expected credit losses	0	0
Corporate bonds
Schedule of Available-for-sale Securities
Less Than 12 Months	227,663	227,663		586,817
Less Than 12 Months	(1,222)	(1,222)		(2,253)
12 Months or More	0	0		5,976
12 Months or More	0	0		(104)
Total	227,663	227,663		592,793
Total	(1,222)	(1,222)		(2,357)
Debt securities, available-for sale, allowance for expected credit losses	1,085	1,085	$ 1,203	0
Debt Securities, Available-for-sale, Change in Present Value, Expense (Reversal)	(100)	(1,824)
Residential mortgage-backed securities
Schedule of Available-for-sale Securities
Less Than 12 Months	59,991	59,991		410,484
Less Than 12 Months	(149)	(149)		(4,074)
12 Months or More	0	0		3,983
12 Months or More	0	0		(43)
Total	59,991	59,991		414,467
Total	(149)	(149)		(4,117)
Debt securities, available-for sale, allowance for expected credit losses	0	0
Commercial mortgage-backed securities
Schedule of Available-for-sale Securities
Less Than 12 Months	27,134	27,134		18,250
Less Than 12 Months	(968)	(968)		(105)
12 Months or More	0	0		748
12 Months or More	0	0		(7)
Total	27,134	27,134		18,998
Total	(968)	(968)		(112)
Debt securities, available-for sale, allowance for expected credit losses	0	0
Asset-backed securities
Schedule of Available-for-sale Securities
Less Than 12 Months	6,492	6,492		5,406
Less Than 12 Months	(692)	(692)		(29)
12 Months or More	163	163		920
12 Months or More	(10)	(10)		(19)
Total	6,655	6,655		6,326
Total	(702)	(702)		(48)
Debt securities, available-for sale, allowance for expected credit losses	0	0
Structured securities
Schedule of Available-for-sale Securities
Less Than 12 Months	81,862	81,862		40,979
Less Than 12 Months	(1,852)	(1,852)		(94)
12 Months or More	53,520	53,520		109,880
12 Months or More	(2,172)	(2,172)		(2,571)
Total	135,382	135,382		150,859
Total	(4,024)	(4,024)		$ (2,665)
Debt securities, available-for sale, allowance for expected credit losses	0	$ 0
Debt securities, available-for-sale
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Change in Present Value, Expense (Reversal)	$ (118)