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Long-Term Debt - Contingent Funding Agreements (Details) $ in Millions	May 15, 2025 USD ($)	Mar. 06, 2025 USD ($)	Mar. 08, 2018 USD ($) item	Jun. 30, 2025 USD ($)	Mar. 19, 2025 USD ($)	Dec. 31, 2024 USD ($)
Long-Term Debt
Outstanding principal balances				$ 3,958.6		$ 3,980.4
Senior Notes due 2028 \| Senior notes
Long-Term Debt
Interest rate (as a percent)					4.111%
Long-term debt issued					$ 400.0
4.111% notes payable, due 2028
Long-Term Debt
Interest rate (as a percent)				4.111%
Outstanding principal balances				$ 400.0
3.4% notes payable, due 2025
Long-Term Debt
Interest rate (as a percent)	3.40%					3.40%
Outstanding principal balances						$ 400.0
Repayment of senior notes at or prior to maturity	$ 400.0
2028 Trust
Long-Term Debt
Pre-capitalized trust securities sold			$ 400.0
Contingent funding agreements
Long-Term Debt
Number of contingent funding agreements \| item			2
10-year contingent funding agreement \| 2028 Trust
Long-Term Debt
Term of contingent funding agreement			10 years
Semi-annual put premium (as a percent)			1.275%
10-year contingent funding agreement \| 2028 Trust \| Senior Notes due 2028
Long-Term Debt
Amount of notes issuable under option			$ 400.0
Interest rate (as a percent)			4.111%		4.111%
30-year contingent funding agreement \| 2048 Trust
Long-Term Debt
Term of contingent funding agreement			30 years
Annual put premium (as a percent)			1.58%
30-year contingent funding agreement \| 2048 Trust \| Senior Notes due 2048
Long-Term Debt
Amount of notes issuable under option			$ 350.0
Interest rate (as a percent)			4.682%
30-year contingent funding agreement \| 2055 Trust
Long-Term Debt
Term of contingent funding agreement		30 years
Semi-annual put premium (as a percent)		1.289%
30-year contingent funding agreement \| 2055 Trust \| Senior Notes Due 2055
Long-Term Debt
Amount of notes issuable under option		$ 500.0
Interest rate (as a percent)		5.807%
2048 Trust
Long-Term Debt
Pre-capitalized trust securities sold			$ 350.0
2055 Trust
Long-Term Debt
Pre-capitalized trust securities sold		$ 500.0