Percentage of revenue generated from a single external customer that accounts for 10 percent or more of an entity's revenues.

No definition available.

Name:	us-gaap_EntityWideRevenueMajorCustomerPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Major Customers - Additional Information (Detail)	12 Months Ended
Major Customers - Additional Information (Detail)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Shell Plc [Member]
Entity Wide Portfolio Carrying Amount, Major Customer [Line Items]
Entity wide revenue major customer percentage	20.00%	11.00%	15.00%
Vitol Group [Member]
Entity Wide Portfolio Carrying Amount, Major Customer [Line Items]
Entity wide revenue major customer percentage	13.00%