Debt Schedule (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 3,442.2	$ 3,345.7
Debt Instrument, Unamortized Premium (Discount), Net	2.3	6.0
Debt Issuance Costs, Noncurrent, Net	(34.6)	(34.8)
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 2002 \| 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 Issued in 2014 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	350.0	350.0
Notes Issued in 2015 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	275.0	275.0
Notes Issued in 2019 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	400.0	400.0
Notes Issued in 2019 due 2049 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	450.0	450.0
Notes Issued in 2020 due 2025 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	0.0	500.0
Notes Issued in 2021 due 2051 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	600.0	0.0
Notes Issued 1990 to 1996 \| Medium-term Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	20.5	20.5
Notes Issued in 1998 with 2038 Maturity Date \| Junior Subordinated Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities	203.7	203.7
Notes Issued in 2018 with a 2058 Maturity Date \| Junior Subordinated Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 300.0	$ 300.0

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount, after accumulated amortization, of debt discount (premium).

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Amount, after accumulated amortization, of debt issuance costs classified as noncurrent. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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