Annual report pursuant to Section 13 and 15(d)

Summary of Common Units Received Under Adjustment Agreement (Detail)

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Summary of Common Units Received Under Adjustment Agreement (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Schedule Of Equity Method Investments [Line Items]      
Fair Value of Common Units Received $ 5,012 $ 18,363 $ 11,111
Annual Common Unit Adjustment      
Schedule Of Equity Method Investments [Line Items]      
Date Common Units Received Mar. 29, 2018 Mar. 31, 2017 Mar. 31, 2016
Number of Common Units Received 908,042 1,487,218 753,598
Fair Value of Common Units Received $ 5,012 $ 18,363 $ 11,111