Reinsurance - Schedule of Retained Aggregate Loss and Initial and Current Reinsurance Coverage (Details) $ in Millions	Mar. 31, 2026 USD ($)
Reinsurance Policy, Type [Axis]: 2021 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	$ 671
Current first layer retained loss	654
Initial reinsurance coverage	206
Current reinsurance coverage	49
Reinsurance Policy, Type [Axis]: 2022-1 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	462
Current first layer retained loss	429
Initial reinsurance coverage	196
Current reinsurance coverage	133
Reinsurance Policy, Type [Axis]: 2022-2 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	385
Current first layer retained loss	352
Initial reinsurance coverage	25
Current reinsurance coverage	25
Reinsurance Policy, Type [Axis]: 2022-3 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	317
Current first layer retained loss	307
Initial reinsurance coverage	289
Current reinsurance coverage	109
Reinsurance Policy, Type [Axis]: 2022-4 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	264
Current first layer retained loss	254
Initial reinsurance coverage	36
Current reinsurance coverage	36
Reinsurance Policy, Type [Axis]: 2022-5 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	256
Current first layer retained loss	237
Initial reinsurance coverage	201
Current reinsurance coverage	130
Reinsurance Policy, Type [Axis]: 2023-1 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	360
Current first layer retained loss	347
Initial reinsurance coverage	180
Current reinsurance coverage	141
Reinsurance Policy, Type [Axis]: 2024-1 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	362
Current first layer retained loss	358
Initial reinsurance coverage	270
Current reinsurance coverage	270
Reinsurance Policy, Type [Axis]: 2024-2 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	134
Current first layer retained loss	131
Initial reinsurance coverage	90
Current reinsurance coverage	64
Reinsurance Policy, Type [Axis]: 2025-1 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	354
Current first layer retained loss	354
Initial reinsurance coverage	180
Current reinsurance coverage	180
Reinsurance Policy, Type [Axis]: 2025-2 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	294
Current first layer retained loss	294
Initial reinsurance coverage	28
Current reinsurance coverage	28
Reinsurance Policy, Type [Axis]: 2026-1 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	89
Current first layer retained loss	89
Initial reinsurance coverage	44
Current reinsurance coverage	44
Reinsurance Policy, Type [Axis]: 2026-2 XOL
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	73
Current first layer retained loss	73
Initial reinsurance coverage	6
Current reinsurance coverage	6
Reinsurance Policy, Type [Axis]: Mortgage Insurance-Linked Notes
Reinsurance Retention Policy [Line Items]
Current reinsurance coverage	312
Reinsurance Policy, Type [Axis]: Traditional Excess-of-Loss Reinsurance
Reinsurance Retention Policy [Line Items]
Current reinsurance coverage	1,215
Reinsurance Policy, Type [Axis]: Triangle Re 2021-2 Ltd.
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	189
Current first layer retained loss	186
Initial reinsurance coverage	303
Current reinsurance coverage	57
Reinsurance Policy, Type [Axis]: Triangle Re 2021-3 Ltd.
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	304
Current first layer retained loss	297
Initial reinsurance coverage	372
Current reinsurance coverage	88
Reinsurance Policy, Type [Axis]: Triangle Re 2023-1 Ltd.
Reinsurance Retention Policy [Line Items]
Initial first layer retained loss	244
Current first layer retained loss	235
Initial reinsurance coverage	248
Current reinsurance coverage	$ 167