Amount and Nature of Net Assets Restricted from Payment of Dividends (Parenthetical) (Detail) - Fixed Maturities - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
NIC and NSIC
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Restricted net assets, amortized cost	$ 9,856	$ 9,820
NHUK
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Restricted net assets, amortized cost	$ 472,701	$ 457,225