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Fair Value measurements - Summary of Carrying Values and Associated Fair Values of Financial Assets and Liabilities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Carrying Value [Member]    
Financial assets:    
Investments $ 28,798 $ 30,053
Level 2 [Member]    
Financial assets:    
Investments $ 28,735 $ 30,038