v3.26.1
Available-for-Sale Debt Securities (Tables)
 12 Months Ended
Mar. 31, 2026
Investments, Debt and Equity Securities [Abstract]  
Summary of Investments in Debt Securities
Investments in debt securities are as follows:
 
    
Year ended March 31, 2026
         
    
Amortized

cost
     
Gross
unrealized
gains
     
Gross
unrealized
losses
     
Fair value
  
     (In million)      (In million)      (In million)      (In million)  
Available-for-sale
debt securities:
            
U.S. government bonds
    $ 1,273.1      $ 0.6      $ (0.9 )     $ 1,272.8  
UK government bonds
      1,070.5        0.1        (7.2 )       1,063.4  
Other foreign bonds
      1,699.9        0.2        (4.6 )       1,695.5  
Corporate debt securities
      551.7        —         (0.7 )       551.0  
  
 
 
     
 
 
     
 
 
     
 
 
  
Total
Available-for-sale
debt securities
   
$
4,595.2
 
   
$
0.9
 
   
$
(13.4
) 
   
$
4,582.7
 
  
 
 
     
 
 
     
 
 
     
 
 
  
 
    
Year ended March 31, 2025
         
    
Amortized

cost
     
Gross
unrealized
gains
     
Gross
unrealized
losses
     
Fair value
  
     (In million)      (In million)      (In million)      (In million)  
Available-for-sale
debt securities:
            
U.S. government bonds
    $ 1,690.9      $ 3.5      $ (5.4 )     $ 1,689.0  
UK government bonds
      1,088.9        0.7        (7.8 )       1,081.8  
Other foreign bonds
      2,538.1        4.5        (15.8 )       2,526.8  
Corporate debt securities
      717.5        0.1        (1.6 )       716.0  
  
 
 
     
 
 
     
 
 
     
 
 
  
Total
Available-for-sale
debt securities
   
$
6,035.4
 
   
$
8.8
 
   
$
(30.6
) 
   
$
6,013.6
 
  
 
 
     
 
 
     
 
 
     
 
 
  
Summary of Amortized Cost and Fair Value of Securities
The amortized cost and
fair
value of securities
available-for-sale
at March 31, 2026, by contractual maturity, are shown below.
 
    
Amortized

cost
     
Fair value
  
     (In million)      (In million)  
Within one year
    $ 3,632.8      $ 3,630.4  
Due after one year through five years
      962.4        952.3  
  
 
 
     
 
 
  
Total
Available-for-sale
debt securities
   
$
4,595.2
 
   
$
4,582.7
 
  
 
 
     
 
 
  
Summary of Proceeds from Sales, Maturities, Principal Payments Received and Net Realized Gains/(losses)
Proceeds from sales, maturities, principal payments received and net realized gains/(losses) on
available-for-sale
debt securities were as follows for the years ended March 31:
 
    
Year ended March 31,
         
    
2026
     
2025
     
2024
  
     (In million)      (In million)      (In million)  
Proceeds from sales, maturities and principal payments received
    $ 10,807.3      $ 7,514.7      $ 11,823.7  
Gross realized gains
      45.2        10.4        44.2  
Gross realized losses
      (37.6 )       (52.9 )       (47.9 ) 
  
 
 
     
 
 
     
 
 
  
Net realized gains/(losses)
   
$
7.6
 
   
$
(42.5
) 
   
$
(3.7
) 
  
 
 
     
 
 
     
 
 
  
Summary of Unrealized Loss Position
The following tables summarize all
available-for-sale
debt securities in an unrealized loss position for which an allowance for credit losses has not been recorded as at March 31, 2026 and 2025, aggregated by major security type and by length of time such securities have continuously been in an unrealized loss position:
 
           
Less than 12 months
    
12 months or longer
    
Total
  
    
Number of

securities
     
Fair
value
     
Gross
unrealized
loss
    
Fair
value
     
Gross
unrealized
loss
    
Fair
value
     
Gross
unrealized
loss
  
            (In million)     (In million)     (In million)  
March 31, 2026
                   
U.S. government bonds
      26      $ 563.2      $ (0.9 )    $ —       $ —      $ 563.2      $ (0.9 ) 
UK government bonds
      15        728.5        (2.3 )      96.1        (4.9 )      824.6        (7.2 ) 
Other foreign bonds
      67        1,261.1        (4.6 )      —         —        1,261.1        (4.6 ) 
Corporate debt securities
      51        514.4        (0.7 )      —         —        514.4        (0.7 ) 
  
 
 
     
 
 
     
 
 
    
 
 
     
 
 
    
 
 
     
 
 
  
Balance at March 31, 2026
   
 
159
 
   
$
3,067.2
 
   
$
(8.5
) 
  
$
96.1
 
   
$
(4.9
) 
  
$
3,163.3
 
   
$
(13.4
) 
  
 
 
     
 
 
     
 
 
    
 
 
     
 
 
    
 
 
     
 
 
  
           
Less than 12 months
    
12 months or longer
    
Total
  
    
Number of

securities
     
Fair
value
     
Gross
unrealized
loss
    
Fair
value
     
Gross
unrealized
loss
    
Fair
value
     
Gross
unrealized
loss
  
            (In millions)     (In millions)     (In millions)  
March 31, 2025
                   
U.S. government bonds
      46      $ 569.3      $ (5.4 )    $ —       $ —      $ 569.3      $ (5.4 ) 
UK government bonds
      15        166.9        (0.3 )      91.7        (7.5 )      258.6        (7.8 ) 
Other foreign bonds
      97        1,203.4        (9.9 )      282.4        (5.9 )      1,485.8        (15.8 ) 
Corporate debt securities
      38        307.2        (1.6 )      —         —        307.2        (1.6 ) 
  
 
 
     
 
 
     
 
 
    
 
 
     
 
 
    
 
 
     
 
 
  
Balance at March 31, 2025
   
 
196
 
   
$
2,246.8
 
   
$
(17.2
) 
  
$
374.1
 
   
$
(13.4
) 
  
$
2,620.9
 
   
$
(30.6
)