Fair Value Measurements - Schedule of Financial Assets At Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	$ 4,558	$ 29,842
Interest Rate Swaps and Collars
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of derivative instruments, asset derivatives	3,884
Interest Rate Swap Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of derivative instruments, asset derivatives		2,317
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	2,586	2,737
Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	1,972	27,105
Significant Other Observable Inputs (Level 2) | Interest Rate Swaps and Collars
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of derivative instruments, asset derivatives	$ 3,884
Significant Other Observable Inputs (Level 2) | Interest Rate Swap Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of derivative instruments, asset derivatives		$ 2,317