Debt Schedule (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 3,430.4	$ 3,427.8
Debt Instrument, Unamortized Premium (Discount), Net	(2.5)	(2.5)
Debt Issuance Costs, Noncurrent, Net	(30.6)	(33.2)
Long-term Debt	3,430.4	3,429.8
Five-Year Term Loan Facility
Debt Instrument
Line of Credit Facility, Fair Value of Amount Outstanding	350.0	350.0
Notes Issued in 1998 with 2028 Maturity Date | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	335.8	335.8
Notes Payable due 2032 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	39.5	39.5
Notes Payable due 2042 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	500.0	500.0
Notes Payable due 2025 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	275.0	275.0
Notes Payable due 2029 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	400.0	400.0
Notes Payable due 2049 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	450.0	450.0
Notes Payable due 2051 | Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	600.0	600.0
Medium Term Notes Payable Due 2028 | Medium-term Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	18.5	18.5
Notes Issued in 1998 with 2038 Maturity Date | Junior Subordinated Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities	189.7	189.7
Notes Payable due 2058 | Junior Subordinated Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities	300.0	300.0
Notes Issued 1990 to 1996 | Medium-term Notes
Debt Instrument
Short-term Debt, outstanding principal	$ 0.0	$ 2.0