Investments - Available-for-Sale Debt Securities by Contractual Maturity (Details) $ in Thousands	Sep. 30, 2020 USD ($)
Amortized Cost
Due in one year or less	$ 59,932
Due after one year through five years	1,137,937
Due after five years through ten years	1,100,741
Due after ten years	315,632
Mortgage-backed securities	1,684,432
Debt Securities	4,298,674
Fair Value
Due in one year or less	60,490
Due after one year through five years	1,205,672
Due after five years through ten years	1,170,940
Due after ten years	323,028
Mortgage-backed securities	1,789,003
Debt Securities	4,549,133
Reciprocal Exchanges
Amortized Cost
Due in one year or less	0
Due after one year through five years	67,247
Due after five years through ten years	63,434
Due after ten years	20,189
Mortgage-backed securities	145,444
Debt Securities	296,314
Fair Value
Due in one year or less	0
Due after one year through five years	68,826
Due after five years through ten years	64,359
Due after ten years	20,231
Mortgage-backed securities	146,708
Debt Securities	300,124
NGHC, excluding Reciprocal Exchanges
Amortized Cost
Due in one year or less	59,932
Due after one year through five years	1,070,690
Due after five years through ten years	1,037,307
Due after ten years	295,443
Mortgage-backed securities	1,538,988
Debt Securities	4,002,360
Fair Value
Due in one year or less	60,490
Due after one year through five years	1,136,846
Due after five years through ten years	1,106,581
Due after ten years	302,797
Mortgage-backed securities	1,642,295
Debt Securities	$ 4,249,009