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Fair Value and Credit Risk of Financial Instruments - Fair Value Measurement of Financial Assets (Detail) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Collateral deposits $ 8,631 $ 8,628
Financial assets fair value disclosure 140,037 71,555
Money Market Funds [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash equivalents 130,821 60,960
Restricted cash 585 1,967
Collateral deposits 8,631 8,628
(Level 1) Quoted Prices in Active Markets for Identical Assets [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Financial assets fair value disclosure 140,037 71,555
(Level 1) Quoted Prices in Active Markets for Identical Assets [Member] | Money Market Funds [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash equivalents 130,821 60,960
Restricted cash 585 1,967
Collateral deposits 8,631 8,628
(Level 2) Significant Other Observable Inputs [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Financial assets fair value disclosure 0 0
(Level 2) Significant Other Observable Inputs [Member] | Money Market Funds [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash equivalents 0 0
Restricted cash 0 0
Collateral deposits 0 0
(Level 3) Significant Unobservable Inputs [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Financial assets fair value disclosure 0 0
(Level 3) Significant Unobservable Inputs [Member] | Money Market Funds [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash equivalents 0 0
Restricted cash 0 0
Collateral deposits $ 0 $ 0