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Investments (Narratives) (Details) - USD ($) $ in Millions	12 Months Ended
Investments (Narratives) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2023
Debt Securities, Available-for-Sale [Line Items]
Available for sale securities, realized gain	$ 439.4	$ 667.0	$ 196.2
Fair value of sold available-for-sale securities	13.3	106.5
Provision for (or reversal of) credit losses	2.1	1.0
Assets held by insurance regulators	21.5	$ 23.1
Self-storage facilities and commercial properties \| Mortgages [Member]
Debt Securities, Available-for-Sale [Line Items]
Provision for (or reversal of) credit losses	$ 0.4
Self-storage facilities and commercial properties \| Mortgages [Member] \| Minimum [Member]
Debt Securities, Available-for-Sale [Line Items]
Mortgage loan interest rate	3.50%
Self-storage facilities and commercial properties \| Mortgages [Member] \| Maximum [Member]
Debt Securities, Available-for-Sale [Line Items]
Mortgage loan interest rate	7.70%