Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Segment	Jun. 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	$ 196,017	$ 173,777	$ 379,136	$ 326,255
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	141,577	114,987	273,125	213,807
Insurance Companies | UK Branch
Segment Reporting Information [Line Items]
Net earned premiums	$ 18,200	$ 25,500	$ 39,100	$ 42,900