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Debt - Principal Terms of Senior and Junior Subordinated Notes (Details) - USD ($) $ in Millions	Dec. 31, 2024	Sep. 30, 2024	Mar. 31, 2024
Senior notes | 2025 Senior Notes
Debt Instrument [Line Items]
Par value	$ 350.0
Stated coupon	3.50%
Effective interest rate	3.67%
Senior notes | 2030 Senior Notes
Debt Instrument [Line Items]
Par value	$ 350.0
Stated coupon	3.30%
Effective interest rate	3.39%
Senior notes | 2034 Senior Notes
Debt Instrument [Line Items]
Par value	$ 400.0	$ 400.0
Stated coupon	5.50%
Effective interest rate	5.64%
Junior subordinated notes | 2059 Junior Subordinated Notes
Debt Instrument [Line Items]
Par value	$ 300.0
Stated coupon	5.875%
Effective interest rate	5.91%
Junior subordinated notes | 2060 Junior Subordinated Notes
Debt Instrument [Line Items]
Par value	$ 275.0
Stated coupon	4.75%
Effective interest rate	4.78%
Junior subordinated notes | 2061 Junior Subordinated Notes
Debt Instrument [Line Items]
Par value	$ 200.0
Stated coupon	4.20%
Effective interest rate	4.23%
Junior subordinated notes | 2064 Junior Subordinated Notes
Debt Instrument [Line Items]
Par value	$ 450.0		$ 450.0
Stated coupon	6.75%
Effective interest rate	6.76%

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