Debt and Financing Arrangements - Schedule of senior notes outstanding (Details) - USD ($) $ in Thousands		Jul. 02, 2019	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Carrying amount			$ 1,871,626	$ 1,733,528
Senior Notes
Debt Instrument [Line Items]
Principal amount			1,890,000
Carrying amount			$ 1,871,626	1,733,528
Senior Notes | 7.350% senior notes due 2034
Debt Instrument [Line Items]
Stated interest rate (percentage)	[1]		7.35%
Principal amount	[1]		$ 300,000
Carrying amount			$ 297,254	297,150
Senior Notes | 5.144% senior notes due 2043
Debt Instrument [Line Items]
Stated interest rate (percentage)	[2]		5.144%
Principal amount	[2]		$ 500,000
Carrying amount			$ 494,831	494,723
Senior Notes | 4.011% senior notes due 2026
Debt Instrument [Line Items]
Stated interest rate (percentage)	[3]		4.011%
Principal amount	[3]		$ 500,000
Carrying amount			$ 496,806	496,417
Senior Notes | 5.031% senior notes due 2046
Debt Instrument [Line Items]
Stated interest rate (percentage)	[4]		5.031%
Principal amount	[4]		$ 450,000
Carrying amount			$ 445,317	445,238
Senior Notes | Watford notes
Debt Instrument [Line Items]
Stated interest rate (percentage)	[5]		6.50%
Principal amount	[5]		$ 140,000
Carrying amount			$ 137,418	0
Variable Interest Entity, Primary Beneficiary | Senior Notes
Debt Instrument [Line Items]
Principal amount		$ 175,000
Proceeds from issuance of senior notes		172,400
Variable Interest Entity, Primary Beneficiary | Senior Notes | Watford notes
Debt Instrument [Line Items]
Stated interest rate (percentage)	[5]		6.50%
Watford Holdings Ltd | Variable Interest Entity, Primary Beneficiary
Debt Instrument [Line Items]
Carrying amount			$ 172,418	$ 0
Watford Holdings Ltd | Variable Interest Entity, Primary Beneficiary | Senior Notes
Debt Instrument [Line Items]
Payments to Acquire Notes Receivable		$ 35,000

[1] Senior notes of Arch Capital issued on May 4, 2004 and due May 1, 2034 (“2034 notes”).

[2] Senior notes of Arch-U.S., a wholly-owned subsidiary of Arch Capital, issued on December 13, 2013 and due November 1, 2043 (“2043 notes”), fully and unconditionally guaranteed by Arch Capital.

[3] Senior notes of Arch Capital Finance LLC (“Arch Finance”), a wholly-owned finance subsidiary of Arch Capital, issued on December 8, 2016 and due December 15, 2026 (“2026 notes”), fully and unconditionally guaranteed by Arch Capital.

[4] Senior notes of Arch Finance issued on December 8, 2016 and due December 15, 2046 (“2046 notes”), fully and unconditionally guaranteed by Arch Capital

[5] Senior notes of Watford issued on July 2, 2019 and due July 2, 2029, reflecting the elimination of amounts owned by Arch-U.S.