Annual report pursuant to Section 13 and 15(d)

Equity Structure and Transactions - Additional Information (Details)

v2.4.1.9
Equity Structure and Transactions - Additional Information (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Equity Structure And Transactions [Line Items]      
Preferred stock, shares authorized 20,000,000us-gaap_PreferredStockSharesAuthorized 20,000,000us-gaap_PreferredStockSharesAuthorized  
Preferred stock, par value $ 0.00001us-gaap_PreferredStockParOrStatedValuePerShare $ 0.00001us-gaap_PreferredStockParOrStatedValuePerShare  
Preferred stock, issued 0us-gaap_PreferredStockSharesIssued 0us-gaap_PreferredStockSharesIssued  
Cash dividends paid $ 30,260us-gaap_PaymentsOfDividends $ 58,846us-gaap_PaymentsOfDividends $ 82,832us-gaap_PaymentsOfDividends
Common Stock, Regular      
Equity Structure And Transactions [Line Items]      
Paid cash dividends, per share $ 0.40us-gaap_CommonStockDividendsPerShareCashPaid
/ us-gaap_StatementClassOfStockAxis
= wti_CommonStockRegularMember
$ 0.36us-gaap_CommonStockDividendsPerShareCashPaid
/ us-gaap_StatementClassOfStockAxis
= wti_CommonStockRegularMember
$ 0.32us-gaap_CommonStockDividendsPerShareCashPaid
/ us-gaap_StatementClassOfStockAxis
= wti_CommonStockRegularMember
Common Stock, Special      
Equity Structure And Transactions [Line Items]      
Paid cash dividends, per share   $ 0.42us-gaap_CommonStockDividendsPerShareCashPaid
/ us-gaap_StatementClassOfStockAxis
= wti_CommonStockSpecialMember
$ 0.79us-gaap_CommonStockDividendsPerShareCashPaid
/ us-gaap_StatementClassOfStockAxis
= wti_CommonStockSpecialMember
Cash dividends paid   $ 31,800us-gaap_PaymentsOfDividends
/ us-gaap_StatementClassOfStockAxis
= wti_CommonStockSpecialMember
$ 59,000us-gaap_PaymentsOfDividends
/ us-gaap_StatementClassOfStockAxis
= wti_CommonStockSpecialMember