| Document and Entity Information Cover | Oct. 29, 2020 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 29, 2020 |
| 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 |
