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Stock-Based Compensation - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jan. 20, 2017
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                 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                 $ 6,800,000 $ 13,800,000 $ 17,600,000        
Income tax benefits $ 879,000 $ (462,000) $ 7,684,000 $ 6,235,000 $ 36,555,000 $ (16,864,000) $ 6,971,000 $ 7,650,000 14,336,000 34,312,000 $ 34,783,000        
Unrecognized compensation expense 11,800,000       18,000,000       $ 11,800,000 $ 18,000,000          
Weighted Average Periods                 1 year 9 months 18 days 1 year 10 months 25 days          
Aggregate fair value of all unvested RSU $ 44,600,000       $ 49,900,000       $ 44,600,000 $ 49,900,000          
Employee Stock Purchase Plan, shares purchased by employees                 45,000 40,000 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                 45,008 40,488          
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                 $ (1,400,000) $ (2,900,000) $ (6,200,000)        
Restricted Stock Units (RSUs)                              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                              
Number of awards, granted                 24,593 35,297 84,000        
Weighted average grant-date fair values per share                 $ 50.01 $ 56.05 $ 44.52        
Total fair value of shares vested                 $ 5,300,000 $ 3,300,000 $ 2,500,000        
Performance-based Equity Awards                              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                              
Number of awards, granted                 247,554 253,333 404,946        
Weighted average grant-date fair values per share                 $ 54.15 $ 55.70 $ 40.77        
Total fair value of shares vested                 $ 15,300,000 $ 29,000,000 $ 11,500,000        
Minimum | 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 | Non-Performance Based Grants                              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                              
Share based payment award, award vesting period                 4 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                 3,192,376            
Number of stock available to be issued 807,624               807,624            
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