Stockholders Equity - Additional Information (Detail) $ / shares in Units, $ in Millions								1 Months Ended	12 Months Ended
	Feb. 16, 2017	Jan. 20, 2017	Dec. 30, 2016 shares	Dec. 29, 2016 $ / shares	Dec. 06, 2016	Sep. 30, 2016 $ / shares	Jul. 15, 2016 $ / shares	Nov. 30, 2015 USD ($)	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Stockholders Equity [Line Items]
Common stock, authorized shares | shares									50,000,000	50,000,000
Common stock, par value									$ 0.10	$ 0.10
Preferred stock, authorized shares | shares									1,000,000	1,000,000
Preferred stock, par value									$ 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
Undistributed Profit | $								$ 141.9
Dividend Capacity | $									$ 3.0
Navigators Insurance Company
Stockholders Equity [Line Items]
Maximum amount available for payment of dividends without prior regulatory approval | $									102.7
Dividend paid | $									$ 5.0	$ 0.0
Installment 1 - FY 2016
Stockholders Equity [Line Items]
Common stock dividend paid per share							$ 0.045
Cash Dividend Paid Date									Jul. 15, 2016
Dividend payable record date									Jun. 20, 2016
Installment 2 - FY 2016
Stockholders Equity [Line Items]
Common stock dividend paid per share						$ 0.045
Cash Dividend Paid Date									Sep. 30, 2016
Dividend payable record date									Aug. 19, 2016
Installment 3 - FY 2016
Stockholders Equity [Line Items]
Common stock dividend paid per share				$ 0.045
Cash Dividend Paid Date									Dec. 29, 2016
Dividend payable record date									Nov. 18, 2016
Subsequent Event [Member]
Stockholders Equity [Line Items]
Stock split ratio		2
Stockholders equity, stock split		We completed a two-for-one stock split on January 20, 2017.
Dividend payable record date	Mar. 03, 2017