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Fair Value and Credit Risk of Financial Instruments - Fair Value Measurement of Financial Assets (Detail) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Mar. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Financial assets fair value disclosure $ 111,233 $ 140,037 $ 152,370
Money Market Funds [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Cash equivalents 102,127 130,821 141,930
Restricted cash 474 585 1,811
Collateral deposits 8,632 8,631 8,629
(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 111,233 140,037 152,370
(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 102,127 130,821 141,930
Restricted cash 474 585 1,811
Collateral deposits 8,632 8,631 8,629
(Level 2) Significant Other Observable Inputs [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Financial assets fair value disclosure 0 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 0
Restricted cash 0 0 0
Collateral deposits 0 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 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 0
Restricted cash 0 0 0
Collateral deposits $ 0 $ 0 $ 0