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Parent Company 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)
Parent Company    
Debt Instrument    
Debt Instrument, Unamortized Premium (Discount), Net 2.3 6.0
Debt Issuance Costs, Noncurrent, Net (34.6) (34.8)
Debt, Long-term and Short-term, Combined Amount 3,442.2 3,345.7
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 0.0 500.0
Notes Issued in 2020 due 2025 | Parent Company | 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 in 2021 due 2051 | Parent Company | 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 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