Annual report pursuant to Section 13 and 15(d)

Significant Accounting Policies - Percentage of Revenue by Major Customers (Details)

v2.4.1.9
Significant Accounting Policies - Percentage of Revenue by Major Customers (Details) (Sales Revenue Net, Customer Concentration Risk)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Shell Trading (US) Co.
      
Entity Wide Revenue Major Customer [Line Items]      
Percentage of receipts 47.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= wti_ShellTradingUSCoMember 		48.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= wti_ShellTradingUSCoMember 		35.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= wti_ShellTradingUSCoMember
ConocoPhillips
      
Entity Wide Revenue Major Customer [Line Items]      
Percentage of receipts     16.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= wti_ConocoPhillipsMember 		[1]
[1] (1) ConocoPhillips split into two separate companies during 2012 and individually were approximately 8% each.