Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Condensed Balance Sheet Statements, Captions [Line Items]
Common stock, shares issued	48,876,882	48,875,167
Treasury Stock, Common, Shares	3,871,598	2,368,194
Common Stock, Shares, Outstanding	45,005,284
Assets:
Debt Securities, Available-for-sale	$ 122,295	$ 157,020
Short-term investments, at cost (approximates fair value)	269,848	289,712
Other Investments	24,971	28,294
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Investments	293,708	312,915
Other assets	92,003	92,317
Assets	2,223,317	8,058,378	$ 8,428,320
Liabilities:
Liabilities	1,137,151	6,862,857
Redeemable noncontrolling interest	252,981	199,402	17,079	$ 19,983
Stockholders' equity (deficit):
Preferred Stock, Value, Issued	0	0
Common stock	489	489
Additional paid-in capital	369,860	331,007
Accumulated other comprehensive income (loss)	8,483	(188,436)	(160,046)
Retained earnings	370,431	683,643
Common stock held in treasury at cost	(33,473)	(28,339)
Total Octave Specialty Group, Inc. stockholders’ equity	715,790	798,364
Total liabilities, redeemable noncontrolling interest and stockholders’ equity	2,223,317	8,058,378
Ambac Financial Group, Inc Parent Company Only [Member]
Assets:
Debt Securities, Available-for-sale	153	0
Short-term investments, at cost (approximates fair value)	151	0
Other Investments	24,971	28,117
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Investments	64,468	92,556
Cash	10,127	9,981
Investment in subsidiaries	645,507	679,150
Deferred Tax Assets, Tax Deferred Expense	1,497	1,497
Other assets	8,750	21,476
Assets	730,349	804,660
Liabilities:
Liabilities	14,559	6,296
Stockholders' equity (deficit):
Preferred Stock, Value, Issued	0	0
Common stock	489	489
Additional paid-in capital	369,860	331,007
Accumulated other comprehensive income (loss)	8,483	(188,436)
Retained earnings	370,431	683,643
Common stock held in treasury at cost	(33,473)	(28,339)
Total Octave Specialty Group, Inc. stockholders’ equity	715,790	798,364	$ 1,361,658	$ 1,252,331
Total liabilities, redeemable noncontrolling interest and stockholders’ equity	730,349	804,660
Short-term Investments [Member]
Assets:
Debt Securities, Available-for-sale	146,442	127,601
Short-term investments, at cost (approximates fair value)	146,434	127,588
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Short-term Investments [Member] | Ambac Financial Group, Inc Parent Company Only [Member]
Assets:
Short-term investments, at cost (approximates fair value)	$ 39,344	$ 64,439