Debt and Financing Arrangements - Schedule of senior notes outstanding (Details) - USD ($) $ in Millions		Dec. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Carrying amount		$ 2,728	$ 2,726
Senior Notes
Debt Instrument [Line Items]
Principal amount		2,750
Carrying amount		$ 2,728	2,726
Senior Notes | 7.350% senior notes due 2034
Debt Instrument [Line Items]
Stated interest rate (percentage)	[1]	7.35%
Principal amount	[1]	$ 300
Carrying amount		$ 298	298
Senior Notes | 5.144% senior notes due 2043
Debt Instrument [Line Items]
Stated interest rate (percentage)	[2]	5.144%
Principal amount	[2]	$ 500
Carrying amount		$ 496	495
Senior Notes | 4.011% senior notes due 2026
Debt Instrument [Line Items]
Stated interest rate (percentage)	[3]	4.011%
Principal amount	[3]	$ 500
Carrying amount		$ 499	498
Senior Notes | 5.031% senior notes due 2046
Debt Instrument [Line Items]
Stated interest rate (percentage)	[4]	5.031%
Principal amount	[4]	$ 450
Carrying amount		$ 446	446
Senior Notes | 3.635% senior notes due 2050
Debt Instrument [Line Items]
Stated interest rate (percentage)	[5]	3.635%
Principal amount	[5]	$ 1,000
Carrying amount		$ 989	$ 989

[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 Arch Capital issued on June 30, 2020 and due June 30, 2050 (“2050 notes”).