Annual report pursuant to Section 13 and 15(d)

Incentive Compensation Plan and Directors Compensation Plan - Additional Information (Details)

v2.4.1.9
Incentive Compensation Plan and Directors Compensation Plan - Additional Information (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Compensation Plan [Line Items]      
Share based compensation performance awards grant performance period 10 years    
Annual incentive awards payment period 90 days    
Percentage of restricted stock units granted not subject to performance criteria   3.00%wti_PercentageOfRestrictedStockUnitsGrantedDuringPeriodNotSubjectToPerformanceCriteria  
Expected vesting month and year     2014-12
Cash based compensation payment period 75 days    
2014 Annual Incentive Plan Award | Chief Executive Officer      
Compensation Plan [Line Items]      
Common stock pre-determined price per share 14.66us-gaap_SharePrice
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Directors Compensation Plan Share-Based Awards | Restricted Shares      
Compensation Plan [Line Items]      
Vesting rights description Vesting occurs upon completion of the specified vesting period and one-third of each grant vests each year over a three-year period.    
Grant vesting period 3 years 3 years 3 years
Adjusted EBITDA and Adjusted EBITDA Margin      
Compensation Plan [Line Items]      
Restricted stock units earning per share, minimum 0.00%wti_NumberOfRestrictedStockUnitsTargetAwardVestedRangeMinimum
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0.00%wti_NumberOfRestrictedStockUnitsTargetAwardVestedRangeMinimum
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Restricted stock units earning per share, maximum 100.00%wti_NumberOfRestrictedStockUnitsTargetAwardVestedRangeMaximum
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= wti_AdjustedEBITDAAndAdjustedEBITDAMarginPerformanceStockAwardsMember
150.00%wti_NumberOfRestrictedStockUnitsTargetAwardVestedRangeMaximum
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Shareholder ​Return ​Performance ​Stock ​Awards      
Compensation Plan [Line Items]      
Restricted stock units earning per share, minimum   0.00%wti_NumberOfRestrictedStockUnitsTargetAwardVestedRangeMinimum
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0.00%wti_NumberOfRestrictedStockUnitsTargetAwardVestedRangeMinimum
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Restricted stock units earning per share, maximum   200.00%wti_NumberOfRestrictedStockUnitsTargetAwardVestedRangeMaximum
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150.00%wti_NumberOfRestrictedStockUnitsTargetAwardVestedRangeMaximum
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Earnings Per Share Targets      
Compensation Plan [Line Items]      
Restricted stock units earning per share, minimum     0.00%wti_NumberOfRestrictedStockUnitsTargetAwardVestedRangeMinimum
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Restricted stock units earning per share, maximum     100.00%wti_NumberOfRestrictedStockUnitsTargetAwardVestedRangeMaximum
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= wti_RestrictedStockUnitsWithEarningPerSharePerformanceConditionsMember