Amount and Nature of Net Assets Restricted from Payment of Dividends (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		$ 567,796	$ 533,930
NIC and NSIC
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets	[1]	12,187	18,148
NIC and NSIC | Fixed Maturities
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		10,655	10,690
NIC and NSIC | Short-Term Investments
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		290	290
NIC and NSIC | Cash [Member]
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		1,242	7,168
NHUK
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets	[2]	555,609	515,782
NHUK | Fixed Maturities
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		471,831	452,053
NHUK | Short-Term Investments
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		$ 83,778	$ 63,729

[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, 2017 and 2016, as presented in the table above, was on deposit with a U.K. bank to comply with the regulatory requirements of the Prudential Regulation Authority for the underwriting activities of the U.K. Branch.

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