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Property, Plant and Equipment - Property, Plant and Equipment (Detail) - USD ($)
$ in Millions
Sep. 30, 2015
Dec. 31, 2014
Property, Plant and Equipment [Line Items]    
Gross property, plant and equipment $ 1,496.7 $ 1,361.7
Less accumulated depreciation 387.5 353.1
Net property, plant and equipment 1,109.2 1,008.6
Land [Member]    
Property, Plant and Equipment [Line Items]    
Gross property, plant and equipment 5.5 5.5
Pipelines and Related Assets [Member]    
Property, Plant and Equipment [Line Items]    
Gross property, plant and equipment 1,085.0 1,081.4
Storage and Delivery Facilities [Member]    
Property, Plant and Equipment [Line Items]    
Gross property, plant and equipment 197.7 165.8
Assets Under Construction [Member]    
Property, Plant and Equipment [Line Items]    
Gross property, plant and equipment 184.3 85.3
Other [Member]    
Property, Plant and Equipment [Line Items]    
Gross property, plant and equipment $ 24.2 $ 23.7