Debt - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Aug. 31, 2015 USD ($)	Oct. 31, 2014 USD ($)
Debt Disclosure [Line Items]
Debt, Long-term and Short-term, Combined Amount	$ 1,440,700,000		$ 4,279,200,000
Aggregate Debt Redemption		$ 1,659,300,000
unamortized deferred financing costs		$ 6,500,000
Other Credit Facilities
Debt Disclosure [Line Items]
Credit facility maximum borrowing capacity	30,000,000
Line of credit facility, amount outstanding	$ 0
Minimum
Debt Disclosure [Line Items]
Debt agreement financial covenant, leverage ratio	3.50
Maximum
Debt Disclosure [Line Items]
Cost and expenses incurred in connection with acquisition	$ 25,000,000
Debt agreement financial covenant, leverage ratio	3.00
Commercial Paper
Debt Disclosure [Line Items]
Debt, Weighted Average Interest Rate	2.34%
Commercial Paper	$ 34,000,000		398,700,000
Senior Notes One Point Eight Seven Five Percent Due Twenty Seventeen [Member] | Senior Notes
Debt Disclosure [Line Items]
Debt, Weighted Average Interest Rate	1.875%
Debt, Long-term and Short-term, Combined Amount	$ 0		350,000,000
Senior Notes Two Point Nine Percent Due Twenty Eighteen [Member] | Senior Notes
Debt Disclosure [Line Items]
Debt, Weighted Average Interest Rate	2.90%
Debt, Long-term and Short-term, Combined Amount	$ 255,300,000		500,000,000
Senior Notes 2.650% due 2019 | Senior Notes
Debt Disclosure [Line Items]
Debt, Weighted Average Interest Rate	2.65%
Debt, Long-term and Short-term, Combined Amount	$ 250,000,000		250,000,000
Senior Notes Two Point Four Five Percent Due Twenty Nineteen [Member] | Senior Notes
Debt Disclosure [Line Items]
Debt, Weighted Average Interest Rate	2.45%
Debt, Long-term and Short-term, Combined Amount	$ 594,400,000		520,700,000
Senior Notes Three Point Six Two Five Percent Due Twenty Twenty [Member] | Senior Notes
Debt Disclosure [Line Items]
Debt, Weighted Average Interest Rate	3.625%
Debt, Long-term and Short-term, Combined Amount	$ 74,000,000		400,000,000
Senior Notes, 5.000% Due 2021 | Senior Notes
Debt Disclosure [Line Items]
Debt, Weighted Average Interest Rate	5.00%
Debt, Long-term and Short-term, Combined Amount	$ 103,800,000		500,000,000
Senior Notes 3.150% due 2022 | Senior Notes
Debt Disclosure [Line Items]
Debt, Weighted Average Interest Rate	3.15%
Debt, Long-term and Short-term, Combined Amount	$ 88,300,000		550,000,000
Senior Notes Four Point Six Five Percent Due Twenty Twenty Five [Member] | Senior Notes
Debt Disclosure [Line Items]
Debt, Weighted Average Interest Rate	4.65%
Debt, Long-term and Short-term, Combined Amount	$ 19,300,000		250,000,000
Revolving Credit Facility [Member]
Debt Disclosure [Line Items]
Remaining availability under Credit Facility	2,437,600,000
Credit facility maximum borrowing capacity				$ 2,500,000,000	$ 2,100,000,000
Revolving Credit Facility [Member] | Maximum
Debt Disclosure [Line Items]
Remaining availability under Credit Facility	$ 1,897,500,000
Revolving Credit Facility [Member] | Long Term Revolving Credit Facility [Member]
Debt Disclosure [Line Items]
Debt, Weighted Average Interest Rate	3.064%
Debt, Long-term and Short-term, Combined Amount	$ 28,400,000		$ 576,800,000