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Cover Page - USD ($)
 12 Months Ended
Dec. 31, 2019
Feb. 18, 2020
Jun. 30, 2019
Entity Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity File Number 1-15202    
Entity Registrant Name W. R. BERKLEY CORP    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 22-1867895    
Entity Address, Address Line One 475 Steamboat Road    
Entity Address, City or Town Greenwich,    
Entity Address, State or Province CT    
Entity Address, Postal Zip Code 06830    
City Area Code 203    
Local Phone Number 629-3000    
Title of 12(b) Security Common Stock, par value $.20 per share    
Trading Symbol WRB    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 9,617,776,032
Entity Common Stock, Shares Outstanding   183,421,709  
Entity Central Index Key 0000011544    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Amendment Flag false    
Documents Incorporated by Reference [Text Block]
Portions of the Company’s definitive proxy statement, which will be filed with the Securities and Exchange Commission within 120 days after December 31, 2019, are incorporated herein by reference in Part III.
    
5.625% Subordinated Debentures due 2053      
Entity Information [Line Items]      
Title of 12(b) Security 5.625% Subordinated Debentures due 2053    
Trading Symbol WRB PRB    
Security Exchange Name NYSE    
5.90% Subordinated Debentures due 2056      
Entity Information [Line Items]      
Title of 12(b) Security 5.90% Subordinated Debentures due 2056    
Trading Symbol WRB PRC    
Security Exchange Name NYSE    
5.75% Subordinated Debentures due 2056      
Entity Information [Line Items]      
Title of 12(b) Security 5.75% Subordinated Debentures due 2056    
Trading Symbol WRB PRD    
Security Exchange Name NYSE    
5.70% Subordinated Debentures due 2058      
Entity Information [Line Items]      
Title of 12(b) Security 5.70% Subordinated Debentures due 2058    
Trading Symbol WRB PRE    
Security Exchange Name NYSE    
5.10% Subordinated Debentures due 2059      
Entity Information [Line Items]      
Title of 12(b) Security 5.10% Subordinated Debentures due 2059    
Trading Symbol WRB PRF    
Security Exchange Name NYSE