Debt Schedule (Detail) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 1998
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 3,767.6	$ 3,465.2
Debt Instrument, Unamortized Discount (Premium), Noncurrent, Net	(129.2)	(131.3)
Debt Issuance Costs, Noncurrent, Net	(36.7)	(37.0)
Short-term debt	0.0	274.6
Debt Instrument, Unamortized Discount, Current	0.0	(0.2)
Debt Issuance Costs, Current, Net	0.0	0.2
Long-term Debt	3,767.6	3,739.8
Notes Issued in 1998 with 2028 Maturity Date | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 335.8	335.8
Notes Issued in 1998 with 2028 Maturity Date | Senior Notes | Minimum
Debt Instrument
Stated Interest Rate of Debt	6.75%
Notes Issued in 1998 with 2028 Maturity Date | Senior Notes | Maximum
Debt Instrument
Stated Interest Rate of Debt	7.25%
Notes Payable due 2032 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 39.5	39.5
Stated Interest Rate of Debt	7.375%
Notes Payable due 2042 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 500.0	500.0
Stated Interest Rate of Debt	5.75%
Notes Payable due 2029 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 400.0	400.0
Stated Interest Rate of Debt	4.00%
Notes Payable due 2049 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 450.0	450.0
Stated Interest Rate of Debt	4.50%
Notes Payable due 2051 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 600.0	600.0
Stated Interest Rate of Debt	4.125%
Notes Payable due 2041 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 400.0	400.0
Stated Interest Rate of Debt	4.046%
Notes Payable due 2054 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 400.0	400.0
Stated Interest Rate of Debt	6.00%
Medium Term Notes Payable Due 2028 | Medium-Term Note
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 18.5	18.5
Stated Interest Rate of Debt	7.19%
Notes Issued in 1998 with 2038 Maturity Date | Junior Subordinated Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 189.7	189.7
Stated Interest Rate of Debt	7.405%		7.405%
Notes Payable due 2058 | Junior Subordinated Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 300.0	300.0
Stated Interest Rate of Debt	6.25%
Notes Payable due 2025 | Senior Notes
Debt Instrument
Short-term debt	$ 0.0	275.0
Stated Interest Rate of Debt	3.875%
5.250% Senior Notes due 2035 | Senior Notes
Debt Instrument
Stated Interest Rate of Debt	5.25%
Debt Instrument, Face Amount	$ 300.0	$ 0.0