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Stock-Based Compensation - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Jan. 20, 2017
Dec. 06, 2016
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Apr. 30, 2013
Apr. 30, 2009
May 31, 2005
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Stock split effective date                     Jan. 20, 2017          
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.                            
Total compensation cost for share-based payment arrangements recognized in income statement                     $ 13,800,000 $ 17,600,000 $ 15,000,000      
Income tax benefits     $ 36,555,000 $ (16,864,000) $ 6,971,000 $ 7,650,000 $ 9,866,000 $ 7,875,000 $ 7,053,000 $ 9,989,000 34,312,000 34,783,000 $ 36,417,000      
Unrecognized compensation expense     18,000,000       22,300,000       18,000,000 $ 22,300,000        
Weighted Average Periods                       1 year 10 months 24 days 1 year 9 months 18 days      
Aggregate fair value of all unvested RSU     $ 49,900,000       $ 87,200,000       $ 49,900,000 $ 87,200,000        
Number of stock options granted                     0 0 0      
Number of stock options outstanding     0               0          
Employee Stock Purchase Plan, shares purchased by employees                     40,488 41,224 42,000      
Employee stock purchase plan                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Exercise price as percentage of fair market value                     90.00%          
Percentage of employees base compensation that can be invested for the purchase of common stock, maximum     10.00%               10.00%          
Number of shares which can be acquired by employee, maximum                     1,000          
Total market value of shares that can be acquired by employee                     $ 25,000          
Employee Stock Purchase Plan, shares purchased by employees                     40,488 41,224        
Discount on employee stock purchase plan                     10.00%          
Share-based Awards                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Income tax benefits                     $ 2,900,000 $ 6,200,000 $ 5,200,000      
Restricted Stock Units (RSUs)                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Number of awards, granted                     35,297 84,000 64,316      
Weighted average grant-date fair values per share                     $ 56.05 $ 44.52 $ 38.72      
Total fair value of shares vested                     $ 3,300,000 $ 2,500,000 $ 2,600,000      
Performance-based Equity Awards                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Number of awards, granted                     253,333 404,946 370,100      
Weighted average grant-date fair values per share                     $ 55.70 $ 40.77 $ 37.25      
Total fair value of shares vested                     $ 29,000,000 $ 11,500,000 $ 12,500,000      
Minimum | Options and Non Performance Based Grants                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Share based payment award, award vesting period                     3 years          
Minimum | Performance Based Shares Issued in 2017, 2016 and 2015 | Third Anniversary of Grant Date                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Percentage of shares vesting over the period                     50.00%          
Maximum | Options and Non Performance Based Grants                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Share based payment award, award vesting period                     4 years          
Options expiration term (in years)                     10 years          
Maximum | Performance Based Shares Issued in 2017, 2016 and 2015 | Third Anniversary of Grant Date                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Percentage of shares vesting over the period                     150.00%          
2005 Stock Incentive Plan                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Number of stock authorized for issuance                               1,000,000
Second Amended and Restated 2005 Stock Incentive Plan                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Number of stock authorized for issuance 4,000,000                         2,000,000    
Stockholders equity, stock split                     The stock split that occurred on January 20, 2017 doubled the number of awards from 2,000,000 to 4,000,000.          
Stock issued during period under incentive plan                     3,156,664          
Number of stock available to be issued     843,336               843,336          
First Amended and Restated 2005 Stock Incentive Plan                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Number of stock authorized for issuance                             1,500,000