Supplementary Insurance Information (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs		$ 119,660	$ 91,983	$ 79,452
Reserve for losses and LAE		2,289,727	2,202,644	2,159,634
Unearned premiums		887,344	820,676	766,167
Net earned premiums		1,100,345	984,087	935,895
Net investment income	[1]	79,413	68,571	64,092
Losses and LAE incurred		665,448	572,598	545,229
Amortization of deferred policy acquisition costs	[2]	165,045	129,977	127,695
Other operating expenses	[1]	230,583	220,643	196,825
Net written premiums		1,186,224	1,043,860	1,000,138
NIC, NSIC and Navigators Management Company, Inc.
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs		83,060	67,144	63,200
Reserve for losses and LAE		1,753,630	1,692,886	1,645,984
Unearned premiums		640,210	607,062	579,331
Net earned premiums		798,151	722,749	704,574
Net investment income	[1]	71,044	61,863	56,714
Losses and LAE incurred		482,789	451,296	434,396
Amortization of deferred policy acquisition costs	[2]	108,809	88,597	85,137
Other operating expenses	[1]	162,387	153,194	138,675
Net written premiums		844,987	757,117	752,773
NHUK
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs		36,600	24,839	16,252
Reserve for losses and LAE		536,097	509,758	513,650
Unearned premiums		247,134	213,614	186,836
Net earned premiums		302,194	261,338	231,321
Net investment income	[1]	8,344	6,708	7,378
Losses and LAE incurred		182,659	121,302	110,833
Amortization of deferred policy acquisition costs	[2]	56,236	41,380	42,558
Other operating expenses	[1]	68,152	67,449	58,150
Net written premiums		341,237	$ 286,743	$ 247,365
NIIC
Supplementary Insurance Information, by Segment [Line Items]
Net investment income	[1]	25
Other operating expenses	[1]	$ 44

[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.