Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Maximum combined ratio indicating underwriting profit									100.00%
Minimum combined ratio indicating underwriting loss									100.00%
Net earned premiums	$ 191,757	$ 173,633	$ 173,777	$ 152,478	$ 166,158	$ 168,233	$ 161,471	$ 164,069	$ 691,645	$ 659,931	$ 683,363
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									472,463	438,851	479,121
UK Branch | Insurance Companies
Segment Reporting Information [Line Items]
Net earned premiums									$ 88,300	$ 86,100	$ 85,400