Amount and Nature of Net Assets Restricted from Payment of Dividends (Detail) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		$ 533,930	$ 541,023
NIC and NSIC
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets	[1]	18,148	17,775
NIC and NSIC | Fixed maturities
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		10,690	11,277
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
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		7,168	6,208
NHUK
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets	[2]	515,782	523,248
NHUK | Fixed maturities
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		452,053	448,666
NHUK | Short-term investments
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		$ 63,729	71,320
NHUK | Cash
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets			$ 3,262

[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, 2016 and 2015, 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.