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, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule Of Equity Method Investments [Line Items]      
Fair Value of Common Units Received $ 18,363 $ 11,111 $ 15,421
Annual Common Unit Adjustment      
Schedule Of Equity Method Investments [Line Items]      
Date Common Units Received Mar. 31, 2017 Mar. 31, 2016 Mar. 31, 2015
Number of Common Units Received 1,487,218 753,598 1,074,910
Fair Value of Common Units Received $ 18,363 $ 11,111 $ 15,421