Parent Company Debt Schedule (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument
Long-term Debt, Excluding Current Maturities	$ 3,345.7	$ 2,926.9
Debt Instrument, Unamortized Premium (Discount), Net	6.0	8.4
Short-term Debt, outstanding principal	0.0
Debt Issuance Costs, Noncurrent, Net	(34.8)	(35.4)
Debt Issuance Costs, Current, Net	0.0	(0.3)
Fair Value Hedge Adjustment, LT Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities	0.0	(0.6)
Parent Company
Debt Instrument
Long-term Debt, Excluding Current Maturities	3,345.7	2,846.9
Debt Instrument, Unamortized Premium (Discount), Net	6.0	8.4
Debt Issuance Costs, Noncurrent, Net	(34.8)	(35.4)
Debt Issuance Costs, Current, Net	0.0	(0.3)
Debt, Long-term and Short-term, Combined Amount	3,345.7	3,246.6
Parent Company \| Fair Value Hedge Adjustment, LT Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities	0.0	(0.6)
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 \| Parent Company \| 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 Issued in 2002 \| Parent Company \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	39.5	39.5
Notes Issued in 2012 and 2016 \| Parent Company \| 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 2014 \| Parent Company \| 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 2015 \| Parent Company \| 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 \| Parent Company \| 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 2019 due 2049 \| Parent Company \| 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	500.0	0.0
Notes Issued in 2020 due 2025 \| Parent Company \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	500.0	0.0
Notes Issued 1990 to 1996 \| Medium-term Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities	20.5	20.5
Notes Issued 1990 to 1996 \| Parent Company \| 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 1998 with 2038 Maturity Date \| Parent Company \| 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
Notes Issued in 2018 with a 2058 Maturity Date \| Parent Company \| Junior Subordinated Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities	300.0	300.0
Notes Issued in 2010 \| Senior Notes
Debt Instrument
Short-term Debt, outstanding principal		400.0
Notes Issued in 2010 \| Senior Notes \| Parent Company
Debt Instrument
Short-term Debt, outstanding principal	$ 0.0	$ 400.0

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

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