Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended				3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011 UK Branch Insurance Companies	Sep. 30, 2010 UK Branch Insurance Companies	Sep. 30, 2011 UK Branch Insurance Companies	Sep. 30, 2010 UK Branch Insurance Companies	Sep. 30, 2011 Navigators Insurance Company	Sep. 30, 2011 Lloyd's Operations	Sep. 30, 2010 Lloyd's Operations	Sep. 30, 2011 Lloyd's Operations	Sep. 30, 2010 Lloyd's Operations	Sep. 30, 2011 Lloyd's Operations Syndicate 1221	Dec. 31, 2010 Lloyd's Operations Syndicate 1221	Sep. 30, 2011 Insurance Companies	Sep. 30, 2010 Insurance Companies	Sep. 30, 2011 Insurance Companies	Sep. 30, 2010 Insurance Companies
Segment Reporting Information [Line Items]
Percentage of Navigators Specialty Insurance Company reinsured by Navigators Insurance Company									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	$ 173,633	$ 168,233	$ 499,888	$ 493,773	$ 21,200	$ 23,300	$ 64,100	$ 61,900		$ 54,301	$ 56,035	$ 166,749	$ 159,939			$ 119,332	$ 112,198	$ 333,139	$ 333,834