Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Segment	Mar. 31, 2012
Segment Reporting Information [Line Items]
Number of business segments	2
Maximum combined ratio indicating underwriting profit	100.00%
Minimum combined ratio indicating underwriting loss	100.00%
Net earned premiums	$ 202,328	$ 183,119
Navigators Insurance Company
Segment Reporting Information [Line Items]
Percentage of Navigators Specialty Insurance Company reinsured by Navigators Insurance Company	100.00%
Insurance
Segment Reporting Information [Line Items]
Net earned premiums	154,331	131,548
Insurance | UK Branch
Segment Reporting Information [Line Items]
Net earned premiums	10,100	21,000
Lloyd's
Segment Reporting Information [Line Items]
Net earned premiums	$ 47,997	$ 51,571
Lloyd's | Syndicate 1221
Segment Reporting Information [Line Items]
Percentage control of stamp capacity	100.00%	100.00%