Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended				3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011 UK Branch Insurance Companies	Jun. 30, 2010 UK Branch Insurance Companies	Jun. 30, 2011 UK Branch Insurance Companies	Jun. 30, 2010 UK Branch Insurance Companies	Jun. 30, 2011 Navigators Insurance Company	Jun. 30, 2011 Lloyd's Operations	Jun. 30, 2010 Lloyd's Operations	Jun. 30, 2011 Lloyd's Operations	Jun. 30, 2010 Lloyd's Operations	Jun. 30, 2011 Lloyd's Operations Syndicate 1221	Dec. 31, 2010 Lloyd's Operations Syndicate 1221	Jun. 30, 2011 Insurance Companies	Jun. 30, 2010 Insurance Companies	Jun. 30, 2011 Insurance Companies	Jun. 30, 2010 Insurance Companies
Segment Reporting Information [Line Items]
Percentage of Navigators Specialty Insurance Company reinsured by Navigators Insurance Company			74.30%						100.00%
Percentage control of stamp capacity														100.00%	100.00%
Maximum combined ratio indicating underwriting profit			100.00%
Minimum combined ratio indicating underwriting loss			100.00%
Net earned premiums	$ 173,777	$ 161,471	$ 326,255	$ 325,540	$ 25,500	$ 18,700	$ 42,900	$ 38,600		$ 58,790	$ 51,046	$ 112,448	$ 103,904			$ 114,987	$ 110,425	$ 213,807	$ 221,636