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Subsequent Events - Additional Information (Detail) - Subsequent Event [Member]
$ in Millions
Apr. 30, 2018
USD ($)
Subsequent Event [Line Items]  
Net proceeds of debt issuances $ 986.1
Debt issuance costs 11.9
Notes Three Point Nine Five Zero Percent Due Two Thousand Twenty Eight [Member]  
Subsequent Event [Line Items]  
Debt instrument face amount $ 500.0
Debt instrument interest rate 3.95%
Debt instrument - maturity year 2028
Notes Four Point Four Zero Zero Percent Due Two Thousand Forty Eight [Member]  
Subsequent Event [Line Items]  
Debt instrument face amount $ 500.0
Debt instrument interest rate 4.40%
Debt instrument - maturity year 2048
Amended And Restated Revolving Credit Facility [Member]  
Subsequent Event [Line Items]  
Credit facilities, amount outstanding $ 0.0
Line of revolving credit facility expiration date 2023-04