Reinsurance - Coverage and retention (Details) - Variable Interest Entity, Not Primary Beneficiary $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($)
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	$ 1,720
Current Coverage	1,409
Remaining Retention, Net	$ 818
Amortization period	10 years
Bellemeade 2021-3 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	$ 639	[1]
Current Coverage	461	[1]
Remaining Retention, Net	132	[1]
Bellemeade 2021-3 Ltd | Funded By VIE
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	508
Bellemeade 2021-3 Ltd | Directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	131
Bellemeade 2022-1 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	317	[2]
Current Coverage	218	[2]
Remaining Retention, Net	140	[2]
Bellemeade 2022-1 Ltd | Funded By VIE
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	284
Bellemeade 2022-1 Ltd | Directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	33
Bellemeade 2022-2 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	327	[3]
Current Coverage	293	[3]
Remaining Retention, Net	199	[3]
Bellemeade 2022-2 Ltd | Funded By VIE
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	201
Bellemeade 2022-2 Ltd | Directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	126
Bellemeade 2023-1 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	233	[4]
Current Coverage	233	[4]
Remaining Retention, Net	175	[4]
Bellemeade 2023-1 Ltd | Funded By VIE
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	186
Bellemeade 2023-1 Ltd | Directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	47
Bellemeade 2024-1 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	204	[5]
Current Coverage	204	[5]
Remaining Retention, Net	172	[5]
Bellemeade 2024-1 Ltd | Funded By VIE
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	163
Bellemeade 2024-1 Ltd | Directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	$ 41

[1] Issued in September 2021, covering in-force policies issued between April 1, 2021 and June 30, 2021. $508 million was directly funded by Bellemeade Re 2021-3 Ltd. via insurance-linked notes, with an additional $131 million capacity provided directly to Arch MI U.S. by a separate panel of reinsurers

[2] Issued in January 2022, covering in-force policies issued between July 1, 2021 and November 30, 2021. $284 million was directly funded by Bellemeade Re 2022-1 Ltd. via insurance-linked notes, with an additional $33 million capacity provided directly to Arch MI U.S. by a separate panel of reinsurers.

[3] Issued in September 2022, covering in-force policies issued between November 1, 2021 and June 30, 2022. $201 million was directly funded by Bellemeade Re 2022-2 Ltd. via insurance-linked notes, with an additional $126 million capacity provided directly to Arch MI U.S. by a separate panel of reinsurers

[4] Issued in October 2023, covering in-force policies issued between January 1, 2023 and September 30, 2023. $186 million was directly funded by Bellemeade Re 2023-1 Ltd. via insurance-linked notes, with an additional $47 million capacity provided directly to Arch MI U.S. by a separate panel of reinsurers

[5] Issued in August 2024, covering in-force policies issued between September 1, 2023 and July 31, 2024. $163 million was directly funded by Bellemeade Re 2024-1 Ltd. via insurance-linked notes, with an additional $41 million capacity provided directly to Arch MI U.S. by a separate panel of reinsurers