Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Segment	Sep. 30, 2012
Segment Reporting Information [Line Items]
Number of business segments			2
Combined ratio indicating underwriting profit			100.00%
Combined ratio indicating underwriting loss			100.00%
Net earned premiums	$ 213,895	$ 201,262	$ 622,037	$ 580,398
Navigators Insurance Company
Segment Reporting Information [Line Items]
Percentage of Navigators Specialty Insurance Company reinsured by Navigators Insurance Company	100.00%		100.00%
Lloyd's
Segment Reporting Information [Line Items]
Net earned premiums	50,933	52,172	148,197	158,183
Lloyd's | Syndicate 1221
Segment Reporting Information [Line Items]
Percentage control of stamp capacity	100.00%	100.00%	100.00%	100.00%
Insurance
Segment Reporting Information [Line Items]
Net earned premiums	162,962	149,090	473,840	422,215
Insurance | UK Branch
Segment Reporting Information [Line Items]
Net earned premiums	$ 13,900	$ 16,900	$ 36,700	$ 56,000