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Summary of Significant Accounting Policies - Fair Value Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Recurring $ 10,998 $ 156,980
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Recurring 10,998 156,980
Fair Value, Inputs, Level 1 [Member] | Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Recurring 0 0
Fair Value, Inputs, Level 1 [Member] | Derivative [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Recurring 0 0
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Recurring [1] 37,291 40,110
Liabilities, Fair Value Disclosure, Recurring [1] 315 43
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Recurring [1] 0 0
Fair Value, Inputs, Level 2 [Member] | Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Recurring [1] 36,442 39,930
Fair Value, Inputs, Level 2 [Member] | Derivative [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Recurring [1] 849 180
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Recurring 0 0
Liabilities, Fair Value Disclosure, Recurring   0
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Recurring 0 0
Fair Value, Inputs, Level 3 [Member] | Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Recurring 0 0
Fair Value, Inputs, Level 3 [Member] | Derivative [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets, Fair Value Disclosure, Recurring 0 0
Derivative Financial Instruments, Liabilities [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liabilities, Fair Value Disclosure, Recurring 0 0
Derivative Financial Instruments, Liabilities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liabilities, Fair Value Disclosure, Recurring [1] 315 43
Derivative Financial Instruments, Liabilities [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liabilities, Fair Value Disclosure, Recurring $ 0 $ 0
[1] (1)Level 2 fair value estimates are based on observable inputs other than quoted prices in active markets for identical assets and liabilities, quoted prices for identical or similar assets or liabilities in inactive markets, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.