Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 Segment	Dec. 31, 2011	Dec. 31, 2010
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	$ 201,566	$ 201,262	$ 196,017	$ 183,119	$ 191,757	$ 173,633	$ 173,777	$ 152,478	$ 781,964	$ 691,645	$ 659,931
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									571,439	472,463	438,851
Insurance Companies | UK Branch
Segment Reporting Information [Line Items]
Net earned premiums									$ 63,900	$ 88,300	$ 86,100