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Fair Value of Financial Instruments - Summary of Changes in Fair Value of Investments (Detail) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Fair Value Disclosures [Abstract]    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales $ 12 $ 12
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning Balance 686 648
Purchases and adjustments 110 54
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0 24
Investment Income 7 5
Ending Balance $ 791 $ 671