Identifiable Assets by Legal Entity (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Identifiable assets:
Total identifiable assets	$ 4,584,012	$ 4,476,185
NIC, NSIC and Navigators Management Company, Inc
Identifiable assets:
Total identifiable assets	3,455,586	3,356,093
NHUK
Identifiable assets:
Total identifiable assets	996,242	957,795
Parent Company
Identifiable assets:
Total identifiable assets	$ 132,184	$ 162,297