Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Segment	Sep. 30, 2011
Segment Reporting Information [Line Items]
Number of business segments			2
Maximum combined ratio indicating underwriting profit			100.00%
Minimum combined ratio indicating underwriting loss			100.00%
Net earned premiums	$ 201,262	$ 173,633	$ 580,398	$ 499,888
Navigators Insurance Company
Segment Reporting Information [Line Items]
Percentage of Navigators Specialty Insurance Company reinsured by Navigators Insurance Company	100.00%		100.00%
Insurance Companies
Segment Reporting Information [Line Items]
Net earned premiums	149,090	119,332	422,215	333,139
Insurance Companies | UK Branch
Segment Reporting Information [Line Items]
Net earned premiums	$ 16,900	$ 21,200	$ 56,000	$ 64,100