Stockholders Equity - Additional Information (Detail) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 30, 2016 shares	Dec. 06, 2016	Nov. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 $ / shares
Stockholders Equity [Line Items]
Common stock, authorized shares | shares				50,000,000	50,000,000	50,000,000
Common stock, par value | $ / shares				$ 0.10	$ 0.10	$ 0.10
Preferred stock, authorized shares | shares				1,000,000	1,000,000	1,000,000
Preferred stock, par value | $ / shares				$ 0.10	$ 0.10	$ 0.10
Stock split ratio		2
Stockholders equity, stock split					On December 6, 2016, our Board of Directors declared a two-for-one stock split of The Navigators Group, Inc. Common stock, to be effected in the form of a stock dividend.
Number of additional shares for every share held | shares	1
Common stock dividend paid per share | $ / shares					$ 0.28	$ 0.225	$ 0.135
Maximum
Stockholders Equity [Line Items]
Quarterly cash dividend payments restricted per share | $ / shares				$ 0.07
NCUL
Stockholders Equity [Line Items]
Undistributed Profit | $			$ 235.8
NIIC
Stockholders Equity [Line Items]
Maximum amount available for payment of dividends without prior regulatory approval | $				$ 20.3	$ 20.3
Navigators Insurance Company
Stockholders Equity [Line Items]
Maximum amount available for payment of dividends without prior regulatory approval | $				$ 5.2	5.2
Dividend paid | $					$ 97.0	$ 19.0