Long-Term Debt - Long-Term Debt (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Total debt	$ 2,626.5	$ 2,625.8
Long-term debt	2,626.5	2,625.8
4.25% Senior Notes, Due 2024
Debt Instrument [Line Items]
Total debt	397.8	397.6
7% Debentures, Due 2025
Debt Instrument [Line Items]
Total debt	124.5	124.5
3.450% Senior Notes, Due 2027
Debt Instrument [Line Items]
Total debt	297.7	297.6
3.500% Senior Notes, Due 2027
Debt Instrument [Line Items]
Total debt	495.9	495.8
2.500% Senior Notes, Due 2030
Debt Instrument [Line Items]
Total debt	490.4	490.1
6.25% Senior Notes, Due 2037
Debt Instrument [Line Items]
Total debt	228.2	228.2
4.250% Senior Notes, Due 2047
Debt Instrument [Line Items]
Total debt	591.9	591.9
Other Notes
Debt Instrument [Line Items]
Total debt	$ 0.1	$ 0.1