Investments - Available-for-Sale Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities
Amortized Cost	$ 4,298,674		$ 4,376,691
Allowance for Credit Losses	(1,085)	$ (1,203)	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Total	258,701		110,430
Total	(7,157)		(10,763)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	4,549,133		4,476,358
Fair Value	4,549,133		4,476,358
Reciprocal Exchanges
Schedule of Available-for-sale Securities
Amortized Cost	296,314		319,190
Allowance for Credit Losses	(150)
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Total	4,354		5,479
Total	(394)		(420)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	300,124		324,249
Fair Value	300,124		324,249
NGHC, excluding Reciprocal Exchanges
Schedule of Available-for-sale Securities
Amortized Cost	4,002,360		4,057,501
Allowance for Credit Losses	(935)
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Total	254,347		104,951
Total	(6,763)		(10,343)
Debt Securities, Available-for-sale [Abstract]
Fair Value	4,249,009		4,152,109
U.S. Treasury
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	55,630		65,037
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	3,551		1,992
Debt Securities	0		(23)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	59,181		67,006
U.S. Treasury \| Reciprocal Exchanges
Schedule of Available-for-sale Securities
Amortized Cost	14,466		12,929
Debt Securities, Available-for-sale [Abstract]
Fair Value	14,795		13,407
U.S. Treasury \| NGHC, excluding Reciprocal Exchanges
Schedule of Available-for-sale Securities
Amortized Cost	41,164		52,108
Debt Securities, Available-for-sale [Abstract]
Fair Value	44,386		53,599
Federal agencies
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	258		3,907
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	5		8
Debt Securities	0		0
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	263		3,915
States and political subdivision bonds
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	311,092		298,345
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	16,653		4,778
Debt Securities	(92)		(1,441)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	327,653		301,682
Foreign government
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	1,764		1,762
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	98		40
Debt Securities	0		0
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	1,862		1,802
Corporate bonds
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	2,038,791		1,859,736
Allowance for Credit Losses	(1,085)	$ (1,203)	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	131,410		59,184
Debt Securities	(1,222)		(2,357)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	2,167,894		1,916,563
Residential mortgage-backed securities
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	1,078,657		1,265,830
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	47,491		15,747
Debt Securities	(149)		(4,117)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	1,125,999		1,277,460
Commercial mortgage-backed securities
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	572,951		585,044
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	57,865		27,261
Debt Securities	(968)		(112)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	629,848		612,193
Asset-backed securities
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	32,824		74,465
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	1,034		1,194
Debt Securities	(702)		(48)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	33,156		75,611
Structured securities
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	206,707		222,565
Allowance for Credit Losses	0
Debt Securities, Available-for-sale, Unrealized Gain (Loss)
Debt Securities	594		226
Debt Securities	(4,024)		(2,665)
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale	$ 203,277		$ 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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