Company's Repurchase Activities (Detail) - $ / shares shares in Thousands		1 Months Ended				3 Months Ended	72 Months Ended
		Jan. 31, 2019	Dec. 31, 2018	Nov. 30, 2018	Oct. 31, 2018	Dec. 31, 2018	Sep. 30, 2018
Repurchase Plan
Capital Unit [Line Items]
Total Number of Units Purchased			266	182	151	599	7,937
Average Price Paid per Unit	[1]		$ 9.41	$ 9.71	$ 9.70	$ 9.57	$ 7.11
Maximum Number of Units that May Yet Be Purchased			4,760	5,026	5,208	4,760	5,359
Repurchase Plan | Subsequent Event
Capital Unit [Line Items]
Total Number of Units Purchased		516
Average Price Paid per Unit	[1]	$ 9.39
Maximum Number of Units that May Yet Be Purchased	[2]	4,244
Publicly Announced Plans or Programs As Part of Repurchase Plan
Capital Unit [Line Items]
Total Number of Units Purchased			266	182	151	599	5,493
Publicly Announced Plans or Programs As Part of Repurchase Plan | Subsequent Event
Capital Unit [Line Items]
Total Number of Units Purchased		516

[1] Amount includes repurchase costs.

[2] Of the total available for repurchase, approximately 1.7 million are available for repurchase in open market transactions and 2.5 million are available for repurchase in privately-negotiated transactions.