| Document and Entity Information Cover | Sep. 10, 2021 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K/A |
| Amendment Flag | true |
| Amendment Description | This Amendment No. 1 to Current Report on Form 8-K (the “Form 8-K/A”) amends the Current Report on Form 8‑K filed by Arch Capital Group Ltd. (“ACGL”) on August 16, 2021 (the “Original Form 8-K”). Pursuant to Instruction 2 to Item 5.02, this Form 8-K/A is being filed solely to provide the information required by Item 5.02(d)(3), as such information was not determined and was unavailable at the time of the required filing of the Original Form 8-K. No other changes have been made to the Original Form 8-K. |
| Document Period End Date | Sep. 10, 2021 |
| Entity Registrant Name | Arch Capital Group Ltd. |
| Entity Central Index Key | 0000947484 |
| Entity Incorporation, State or Country Code | D0 |
| Entity File Number | 001-16209 |
| Entity Tax Identification Number | 98-0374481 |
| Entity Address, Address Line One | Waterloo House, Ground Floor |
| Entity Address, Address Line Two | 100 Pitts Bay Road |
| Entity Address, City or Town | Pembroke |
| Entity Address, Postal Zip Code | HM 08 |
| Entity Address, Country | BM |
| City Area Code | 441 |
| Local Phone Number | 278-9250 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common shares | |
| Entity Listings [Line Items] | |
| Title of 12(b) Security | Common shares, $0.0011 par value per share |
| Trading Symbol | ACGL |
| Security Exchange Name | NASDAQ |
| Depositary Series E Preferred Shares | |
| Entity Listings [Line Items] | |
| Title of 12(b) Security | Depositary shares, each representing a 1/1,000th interest in a 5.25% Series E preferred share |
| Trading Symbol | ACGLP |
| Security Exchange Name | NASDAQ |
| Depositary Series F Preferred Shares | |
| Entity Listings [Line Items] | |
| Title of 12(b) Security | Depositary shares, each representing a 1/1,000th interest in a 5.45% Series F preferred share |
| Trading Symbol | ACGLO |
| Security Exchange Name | NASDAQ |
| Depositary Series G Preferred Shares | |
| Entity Listings [Line Items] | |
| Title of 12(b) Security | Depositary shares, each representing a 1/1,000th interest in a 4.55% Series G preferred share |
| Trading Symbol | ACGLN |
| Security Exchange Name | NASDAQ |
