Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Maximum combined ratio indicating underwriting profit	100.00%
Minimum combined ratio indicating underwriting loss	100.00%
Net earned premiums	$ 183,119	$ 152,478
Navigators Insurance Company
Segment Reporting Information [Line Items]
Percentage of Navigators Specialty Insurance Company reinsured by Navigators Insurance Company	100.00%
Insurance Companies
Segment Reporting Information [Line Items]
Net earned premiums	131,548	98,820
Insurance Companies | UK Branch
Segment Reporting Information [Line Items]
Net earned premiums	$ 21,000	$ 17,400