| Document and Entity Information | Mar. 16, 2021 |
| Document And Entity Information [Line Items] | |
| Entity Registrant Name | BERKLEY W R CORP |
| Entity Address, State or Province | CT |
| Amendment Flag | false |
| Entity Central Index Key | 0000011544 |
| Document Type | 8-K |
| Document Period End Date | Mar. 16, 2021 |
| Entity File Number | 1-15202 |
| Entity Incorporation State 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, Postal Zip Code | 06830 |
| City Area Code | (203) |
| Local Phone Number | 629-3000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common Stock, par value $.20 per share |
| Trading Symbol | WRB |
| Security Exchange Name | NYSE |
| M 5.750 Subordinated Debentures Due 2056 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 5.750% Subordinated Debentures due 2056 |
| Trading Symbol | WRB-PD |
| Security Exchange Name | NYSE |
| M 5.700 Subordinated Debentures Due 2058 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 5.700% Subordinated Debentures due 2058 |
| Trading Symbol | WRB-PE |
| Security Exchange Name | NYSE |
| M 5.100 Subordinated Debentures Due 2059 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 5.100% Subordinated Debentures due 2059 |
| Trading Symbol | WRB-PF |
| Security Exchange Name | NYSE |
| M 4.250 Subordinated Debentures Due 2060 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 4.250% Subordinated Debentures due 2060 |
| Trading Symbol | WRB-PG |
| Security Exchange Name | NYSE |
| M 4.125 Subordinated Debentures Due 2061 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 4.125% Subordinated Debentures due 2061 |
| Trading Symbol | WRB-PH |
| Security Exchange Name | NYSE |
