Stockholders Equity - Additional Information (Detail)				1 Months Ended	12 Months Ended
	Jan. 20, 2017	Dec. 30, 2016 shares	Dec. 06, 2016	Nov. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)
Stockholders Equity [Line Items]
Common stock, authorized shares | shares					50,000,000	50,000,000
Common stock, par value | $ / shares					$ 0.10	$ 0.10
Preferred stock, authorized shares | shares					1,000,000	1,000,000
Preferred stock, par value | $ / shares					$ 0.10	$ 0.10
Stock split ratio	2		2
Stockholders equity, stock split	We completed a two-for-one stock split on January 20, 2017.		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					$ 0.225	$ 0.135
Dividend paid							$ 0
NCUL
Stockholders Equity [Line Items]
Undistributed Profit				$ 218,300,000
NIIC
Stockholders Equity [Line Items]
Maximum amount available for payment of dividends without prior regulatory approval					$ 8,400,000
Navigators Insurance Company
Stockholders Equity [Line Items]
Dividend paid					19,000,000	$ 5,000,000
Maximum amount available for payment of dividends without prior regulatory approval					$ 105,700,000