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Investments (Narratives) (Details) - USD ($)
$ in Millions
12 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Mar. 31, 2022
Debt Securities, Available-for-Sale [Line Items]      
Available for sale securities, realized gain $ 1.9 $ 0.9 $ 9.5
Fair value of sold available-for-sale securities 667.0 196.2 352.3
Gross realized gains on sales of Treasuries 1.0 2.0  
Available for sale securities, gross realized losses 3.1 0.3 $ 1.4
Available for sale investments, fair value, recovery amount 0.5 0.5  
Assets held by insurance regulators $ 23.1 $ 23.4  
Mortgages [Member] | Minimum [Member]      
Debt Securities, Available-for-Sale [Line Items]      
Mortgage loan interest rate 3.50%    
Mortgages [Member] | Maximum [Member]      
Debt Securities, Available-for-Sale [Line Items]      
Mortgage loan interest rate   7.70%