Amount and Nature of Net Assets Restricted from Payment of Dividends (Detail) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		$ 597,417	$ 567,482
NIC and NSIC
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets	[1]	16,515	13,773
NIC and NSIC | Fixed Maturities
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		10,441	10,655
NIC and NSIC | Cash
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets	[2]	6,074	3,118
NHUK
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets	[3]	580,889	553,709
NHUK | Fixed Maturities
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		474,128	471,831
NHUK | Cash
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets	[2]	60,233	53,111
NHUK | Other Invested Assets
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets	[2]	46,528	$ 28,767
All other Entities
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		13
All other Entities | Cash
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		$ 13

[1] The restricted net assets for NIC and NSIC primarily consist of fixed maturities on deposit with various state insurance departments. The cash as of December 31, 2018 and 2017, as presented in the table above, is held in various collateral accounts to support underwriting activities.

[2] Cash and overseas deposits that were misclassified within Short-Term Investments prior to 2018 were reclassified, representing an immaterial correction. Refer to Note 1 Organization & Summary of Significant Accounting Policies for further information.

[3] - The restricted net assets for NHUK consists of fixed maturities and cash held in trust for the benefit of syndicate policyholders and other invested assets primarily consisting of overseas deposits in various countries with Lloyd's to support underwriting activities in those countries