Condensed Financial Information - Balance Sheets (Parenthetical) (Detail)	Jan. 20, 2017	Dec. 06, 2016	Dec. 31, 2016 $ / shares shares	Dec. 31, 2015 $ / shares shares
Condensed Financial Statements Captions [Line Items]
Preferred stock, par value | $ / shares			$ 0.10	$ 0.10
Preferred stock, authorized shares			1,000,000	1,000,000
Preferred stock, issued shares			0	0
Common stock, par value | $ / shares			$ 0.10	$ 0.10
Common stock, authorized shares			50,000,000	50,000,000
Common stock, issued shares			36,146,899	35,884,538
Treasury stock, shares			7,022,760	7,022,760
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.
Subsequent Event [Member]
Condensed Financial Statements Captions [Line Items]
Stock split ratio	2
Stockholders equity, stock split	We completed a two-for-one stock split on January 20, 2017.
Parent Company
Condensed Financial Statements Captions [Line Items]
Preferred stock, par value | $ / shares			$ 10	$ 10
Preferred stock, authorized shares			1,000,000	1,000,000
Preferred stock, issued shares			0	0
Common stock, par value | $ / shares			$ 10	$ 10
Common stock, authorized shares			50,000,000	50,000,000
Common stock, issued shares			36,146,899	35,884,538
Treasury stock, shares			7,022,760	7,022,760
Parent Company | Subsequent Event [Member]
Condensed Financial Statements Captions [Line Items]
Stock split ratio	2
Stockholders equity, stock split	We completed a two-for-one stock split on January 20, 2017.