Debt Schedule (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 2,926.9	$ 2,971.3
Debt Instrument, Unamortized Premium (Discount), Net	8.4	9.6
Debt Issuance Costs, Noncurrent, Net	(35.4)	(30.8)
Short-term debt	399.7	0.0
Debt Issuance Costs, Current, Net	(0.3)	0.0
Long-term Debt	3,326.6	2,971.3
Short-term Debt, outstanding principal	400.0
Fair Value Hedge Adjustment, LT Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities	(0.6)	(5.1)
Secured Notes Issued in 2007 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	80.0	140.0
Notes Issued in 1998 with 2028 Maturity Date \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	335.8	365.8
Notes Issued in 2002 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	39.5	39.5
Notes Issued in 2010 \| Senior Notes
Debt Instrument
Short-term Debt, outstanding principal		0.0
Notes Issued in 2010 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	0.0	400.0
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 2016 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	0.0	350.0
Notes Issued in 2019 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	400.0	0.0
Notes Issued in 2019 due 2049 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	450.0	0.0
Notes Issued 1990 to 1996 \| Medium-term Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	20.5	50.8
Notes Issued in 1998 with 2038 Maturity Date \| Junior Subordinated Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities	203.7	226.5
Notes Issued in 2018 with a 2058 Maturity Date \| Junior Subordinated Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 300.0	$ 300.0

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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 current. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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