Debt - Debt Outstanding and Average Interest Rates (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Line Items]
Unamortized Debt Issuance Expense	$ 6.8	$ 17.8
Long-term Debt, Excluding Current Maturities	1,440.7	4,278.4
Less: Current maturities and short-term borrowings	0.0	(0.8)
Total debt	1,440.7	4,279.2
Senior Notes One Point Eight Seven Five Percent Due Twenty Seventeen [Member] | Senior Notes
Debt Disclosure [Line Items]
Total debt	0.0	350.0
Senior Notes Two Point Nine Percent Due Twenty Eighteen [Member] | Senior Notes
Debt Disclosure [Line Items]
Total debt	255.3	500.0
Senior Notes 2.650% due 2019 | Senior Notes
Debt Disclosure [Line Items]
Total debt	250.0	250.0
Senior Notes Two Point Four Five Percent Due Twenty Nineteen [Member] | Senior Notes
Debt Disclosure [Line Items]
Total debt	594.4	520.7
Senior Notes Three Point Six Two Five Percent Due Twenty Twenty [Member] | Senior Notes
Debt Disclosure [Line Items]
Total debt	74.0	400.0
Senior Notes, 5.000% Due 2021 | Senior Notes
Debt Disclosure [Line Items]
Total debt	103.8	500.0
Senior Notes 3.150% due 2022 | Senior Notes
Debt Disclosure [Line Items]
Total debt	88.3	550.0
Senior Notes Four Point Six Five Percent Due Twenty Twenty Five [Member] | Senior Notes
Debt Disclosure [Line Items]
Total debt	19.3	250.0
Other Debt Obligations [Member] | Other Debt Obligations [Member]
Debt Disclosure [Line Items]
Total debt	0.0	0.8
Revolving Credit Facility [Member] | Long Term Revolving Credit Facility [Member]
Debt Disclosure [Line Items]
Total debt	$ 28.4	$ 576.8