Dividends and Statutory Financial Information - Additional Information (Detail)	1 Months Ended				12 Months Ended			12 Months Ended
	Nov. 30, 2013 USD ($)	Nov. 30, 2013 GBP (£)	Dec. 31, 2014 USD ($)	Dec. 31, 2014 GBP (£)	Dec. 31, 2012 Navigators Insurance Company USD ($)	Dec. 31, 2014 Navigators Insurance Company USD ($)	Dec. 31, 2013 Navigators Insurance Company	Dec. 31, 2014 Insurance Companies USD ($)	Dec. 31, 2013 Insurance Companies USD ($)	Dec. 31, 2012 Insurance Companies USD ($)	Dec. 31, 2014 Insurance Companies Minimum	Dec. 31, 2014 Insurance Companies Maximum	Dec. 31, 2014 Lloyd's USD ($)	Dec. 31, 2013 Lloyd's USD ($)	Dec. 31, 2012 Lloyd's USD ($)	Dec. 31, 2014 Lloyd's GBP (£)	Dec. 31, 2013 Lloyd's GBP (£)	Dec. 31, 2014 Lloyd's Syndicate 1221 USD ($)	Dec. 31, 2014 Lloyd's Syndicate 1221 GBP (£)	Dec. 31, 2013 Lloyd's Syndicate 1221 USD ($)	Dec. 31, 2013 Lloyd's Syndicate 1221 GBP (£)	Dec. 31, 2012 Lloyd's Syndicate 1221
Statutory Accounting Practices [Line Items]
Maximum amount available for payment of dividends without prior regulatory approval						$ 89,400,000
Dividends paid to Parent Company					15,000,000
Statutory net income								75,700,000	59,400,000	28,600,000
Statutory capital and surplus								893,900,000	804,100,000				140,100,000	124,200,000		89,400,000	74,800,000
Risk based capital ratio						314.80%	304.20%
Insurance examinations period											3 years	5 years
FAL requirement set by Lloyd's for Syndicate 1221 based on its business plan, approved in November 2014 and 2013													268,100,000	279,300,000		171,100,000	168,300,000
Valuation of assets and letters of credit posted																		290,100,000	185,100,000	280,200,000	168,800,000
Pre-tax regulatory income													33,800,000	21,000,000	54,900,000
Percentage control of stamp capacity																		100.00%	100.00%	100.00%	100.00%	100.00%
Undistributed Profit	139,900,000	89,300,000
Dividend Capacity			$ 9,900,000	£ 6,300,000