Dividends and Statutory Financial Information - Additional Information (Detail)	1 Months Ended				12 Months Ended
	Nov. 30, 2012 USD ($)	Nov. 30, 2012 GBP (£)	Dec. 31, 2012 USD ($)	Dec. 31, 2012 GBP (£)	Dec. 31, 2012 Navigators Insurance Company USD ($)	Dec. 31, 2011 Navigators Insurance Company USD ($)	Dec. 31, 2010 Navigators Insurance Company USD ($)	Dec. 31, 2012 Insurance Companies USD ($)	Dec. 31, 2011 Insurance Companies USD ($)	Dec. 31, 2010 Insurance Companies USD ($)	Dec. 31, 2012 Insurance Companies Minimum	Dec. 31, 2012 Insurance Companies Maximum	Dec. 31, 2012 Lloyd's Operations USD ($)	Dec. 31, 2011 Lloyd's Operations USD ($)	Dec. 31, 2010 Lloyd's Operations USD ($)	Dec. 31, 2012 Lloyd's Operations GBP (£)	Dec. 31, 2011 Lloyd's Operations GBP (£)	Dec. 31, 2012 Lloyd's Operations Syndicate 1221 USD ($)	Dec. 31, 2012 Lloyd's Operations Syndicate 1221 GBP (£)	Dec. 31, 2011 Lloyd's Operations Syndicate 1221 USD ($)	Dec. 31, 2011 Lloyd's Operations Syndicate 1221 GBP (£)	Dec. 31, 2010 Lloyd's Operations Syndicate 1221
Statutory Accounting Practices [Line Items]
Maximum amount available for payment of dividends without prior regulatory approval					$ 68,300,000
Dividends paid to Parent Company					15,000,000	45,000,000	40,000,000
Statutory net income								28,600,000	16,300,000	85,700,000
Statutory capital and surplus								682,900,000	662,200,000				110,000,000	68,200,000		67,500,000	44,000,000
Risk based capital ratio					295.10%	307.80%
Insurance examinations period											3 years	5 years
FAL requirement set by Lloyd's for Syndicate 1221 based on its business plan, approved in November 2012 and 2011													223,400,000	223,400,000		137,100,000	144,100,000
Valuation of assets and letters of credit posted																		227,300,000	139,500,000	227,300,000	146,700,000
Pre-tax regulatory income													54,900,000	5,300,000	6,500,000
Percentage control of stamp capacity																		100.00%	100.00%	100.00%	100.00%	100.00%
Undistributed Profit	91,100,000	55,900,000
Dividend Capacity			$ 8,600,000	£ 5,300,000