Supplementary Insurance Information (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs		$ 91,983	$ 79,452	$ 67,007
Reserve for losses and LAE		2,202,644	2,159,634	2,045,071
Unearned premiums		820,676	766,167	714,606
Net earned premiums		984,087	935,895	841,939
Net investment income	[1]	68,571	64,092	56,243
Losses and LAE incurred		572,598	545,229	518,961
Amortization of deferred policy acquisition costs	[2]	129,977	127,695	115,842
Other operating expenses	[3]	220,643	196,825	164,434
Net written premiums		1,043,860	1,000,138	887,922
NIC, NSIC and Navigators Management Company, Inc.
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs		67,144	63,200	54,984
Reserve for losses and LAE		1,692,886	1,645,984	1,523,175
Unearned premiums		607,062	579,331	536,303
Net earned premiums		722,749	704,574	639,338
Net investment income	[1]	61,863	56,714	49,083
Losses and LAE incurred		451,296	434,396	415,413
Amortization of deferred policy acquisition costs	[2]	88,597	85,137	81,132
Other operating expenses	[3]	153,194	138,675	119,920
Net written premiums		757,117	752,773	680,008
NHUK
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs		24,839	16,252	12,023
Reserve for losses and LAE		509,758	513,650	521,896
Unearned premiums		213,614	186,836	178,303
Net earned premiums		261,338	231,321	202,601
Net investment income	[1]	6,708	7,378	7,160
Losses and LAE incurred		121,302	110,833	103,548
Amortization of deferred policy acquisition costs	[2]	41,380	42,558	34,710
Other operating expenses	[3]	67,449	58,150	44,514
Net written premiums		$ 286,743	$ 247,365	$ 207,914

[1] - Net investment income and Other operating expenses reflect only such amounts attributable to our Company's insurance operations.

[2] Amortization of deferred policy acquisition costs reflects only such amounts attributable to our Company's insurance operations. A portion of these costs is eliminated in consolidation.

[3] Net investment income and Other operating expenses reflect only such amounts attributable to our Company's insurance operations.