Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Segment	Jun. 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	$ 205,814	$ 196,017	$ 408,142	$ 379,136
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	49,267	54,440	97,264	106,011
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	156,547	141,577	310,878	273,125
Insurance | UK Branch
Segment Reporting Information [Line Items]
Net earned premiums	$ 12,700	$ 18,200	$ 22,800	$ 39,100