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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

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