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Derivative instruments - Summarizes Gross Asset and Liability Amounts Related to Derivatives (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Derivatives, Fair Value [Line Items]    
Gross Amount of Assets Recognized as Liabilities $ 1 $ 1
Gross Amounts Offset in the Statement of Financial Position (84) (156)
Net Amounts of Liabilities Presented in the Statement of Financial Position (83) (155)
Interest rate swaps [Member]
   
Derivatives, Fair Value [Line Items]    
Gross Amounts Offset in the Statement of Financial Position (84) (156)
Net Amounts of Liabilities Presented in the Statement of Financial Position (84) (156)
Interest rate caps [Member]
   
Derivatives, Fair Value [Line Items]    
Gross Amount of Assets Recognized as Liabilities 1 1
Net Amounts of Liabilities Presented in the Statement of Financial Position $ 1 $ 1