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Fair Value of Financial Instruments
 9 Months Ended
Sep. 30, 2025
Fair Value Disclosures [Abstract]  
Fair Value of Financial Instruments Fair Value of Financial Instruments
    White Mountains records its financial instruments at fair value with the exception of debt obligations, which are recorded as debt at face value less unamortized debt issuance costs and original issue discount. See Note 7 — “Debt.”
    The following table presents the fair value and carrying value of these financial instruments as of September 30, 2025 and December 31, 2024:
 September 30, 2025December 31, 2024
MillionsFair ValueCarrying ValueFair ValueCarrying Value
Ark 2021 Subordinated Notes$179.6 $159.7 $173.9 $154.5 
HG Global Senior Notes$154.0 $147.7 $157.2 $147.4 
Kudu Credit Facility$289.4 $274.3 $253.3 $238.6 
Bamboo Credit Facility$106.6 $104.6 $— $— 
Distinguished Credit Facility
$139.6 $138.1 $— $— 
Distinguished other debt
$10.6 $10.6 $— $— 
Other Operations debt$36.2 $35.4 $23.1 $22.0 

The fair value estimates for White Mountains’s debt obligations have been determined based on discounted cash flow analyses and are considered to be Level 3 measurements.
For the fair value measurements associated with White Mountains’s investment securities see Note 3 — “Investment Securities.” For the fair value measurements associated with White Mountains’s derivative instruments see Note 9 — “Derivatives.” For the fair value measurements associated with the BAM Surplus Notes see Note 10 — “Municipal Bond Guarantee Reinsurance.”