Common Unit Repurchase and Retirement - Company's Repurchase Activities (Detail) - $ / shares shares in Thousands		1 Months Ended					3 Months Ended		72 Months Ended
		Apr. 30, 2019	Mar. 31, 2019	Feb. 28, 2019		Jan. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
Repurchase Plan
Capital Unit [Line Items]
Total Number of Units Purchased			197	1,474		516	2,187	599	7,937
Average Price Paid per Unit	[1]		$ 9.74	$ 9.41		$ 9.39	$ 9.44	$ 9.57	$ 7.11
Maximum Number of Units that May Yet Be Purchased			2,573	2,770	[2]	4,244	2,573	4,760	5,359
Repurchase Plan | Subsequent Event
Capital Unit [Line Items]
Total Number of Units Purchased		222
Average Price Paid per Unit	[1]	$ 9.67
Maximum Number of Units that May Yet Be Purchased	[3]	2,351
Publicly Announced Plans or Programs As Part of Repurchase Plan
Capital Unit [Line Items]
Total Number of Units Purchased			197	1,474		516	2,187	599	5,493
Publicly Announced Plans or Programs As Part of Repurchase Plan | Subsequent Event
Capital Unit [Line Items]
Total Number of Units Purchased		222

[1] Amount includes repurchase costs.

[2] February 2019 common units repurchased include 1.2 million common units acquired in a private transaction.

[3] Of the total available for repurchase, approximately 1.1 million are available for repurchase in open market transactions and 1.3 million are available for repurchase in privately-negotiated transactions.