Investments - Available-for-Sale Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities
Amortized Cost	$ 4,329,066		$ 4,376,691
Allowance for Credit Losses	(1,203)	$ (2,927)	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Total	265,019		110,430
Total	(11,351)		(10,763)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	4,581,531		4,476,358
Fair Value	4,581,531		4,476,358
Reciprocal Exchanges
Schedule of Available-for-sale Securities
Amortized Cost	304,717		319,190
Allowance for Credit Losses	(149)
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Total	16,735		5,479
Total	(530)		(420)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	320,773		324,249
Fair Value	320,773		324,249
NGHC, excluding Reciprocal Exchanges
Schedule of Available-for-sale Securities
Amortized Cost	4,024,349		4,057,501
Allowance for Credit Losses	(1,054)
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Total	248,284		104,951
Total	(10,821)		(10,343)
Debt Securities, Available-for-sale [Abstract]
Fair Value	4,260,758		4,152,109
U.S. Treasury
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	56,667		65,037
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	4,356		1,992
Debt Securities	(1)		(23)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	61,022		67,006
U.S. Treasury \| Reciprocal Exchanges
Schedule of Available-for-sale Securities
Amortized Cost	12,882		12,929
Debt Securities, Available-for-sale [Abstract]
Fair Value	13,858		13,407
U.S. Treasury \| NGHC, excluding Reciprocal Exchanges
Schedule of Available-for-sale Securities
Amortized Cost	43,785		52,108
Debt Securities, Available-for-sale [Abstract]
Fair Value	47,164		53,599
Federal agencies
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	558		3,907
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	7		8
Debt Securities	0		0
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	565		3,915
States and political subdivision bonds
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	290,985		298,345
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	14,483		4,778
Debt Securities	(135)		(1,441)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	305,333		301,682
Foreign government
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	1,763		1,762
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	82		40
Debt Securities	0		0
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	1,845		1,802
Corporate bonds
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	2,039,050		1,859,736
Allowance for Credit Losses	(1,203)	$ (2,927)	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	136,824		59,184
Debt Securities	(1,232)		(2,357)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	2,173,439		1,916,563
Residential mortgage-backed securities
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	1,096,583		1,265,830
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	53,893		15,747
Debt Securities	(14)		(4,117)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	1,150,462		1,277,460
Commercial mortgage-backed securities
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	590,047		585,044
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	54,361		27,261
Debt Securities	(989)		(112)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	643,419		612,193
Asset-backed securities
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	43,015		74,465
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	961		1,194
Debt Securities	(979)		(48)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	42,997		75,611
Structured securities
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	210,398		222,565
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	52		226
Debt Securities	(8,001)		(2,665)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	$ 202,449		$ 220,126

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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