Debt and Financing Arrangements - Letter of Credit and Revolving Credit Facilities (Details) - USD ($) $ in Thousands	Dec. 17, 2019	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Outstanding borrowings		$ 484,287	$ 455,682
Investments pledged as collateral		255,400
Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 750,000
Credit facility term	5 years
Secured letter of credit facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 250,000
Optional increased capacity		1,250,000
Remaining capacity		24,600
Outstanding borrowings		225,400
Unsecured revolving loan and letter of credit facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 500,000
Arch Re US | Revolving Credit Facility
Line of Credit Facility [Line Items]
Outstanding borrowings		0	0
Other Arch Capital Subsidiaries | Secured letter of credit facility
Line of Credit Facility [Line Items]
Outstanding borrowings		18,100
Other Arch Capital Subsidiaries | Unsecured revolving loan and letter of credit facility
Line of Credit Facility [Line Items]
Outstanding borrowings		195,000
Watford Holdings Ltd
Line of Credit Facility [Line Items]
Outstanding borrowings		122,900
Watford Holdings Ltd | Secured letter of credit facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity		100,000
Watford Holdings Ltd | Unsecured revolving loan and letter of credit facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity		100,000
Watford Holdings Ltd | Secured credit facility expiring November 2021
Line of Credit Facility [Line Items]
Maximum borrowing capacity		800,000
Outstanding borrowings		484,287	$ 455,682
Letter of credit limit		$ 400,000