Stock Transactions and Unit Redemptions - Unit Redemption and Share Repurchase Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Class of Stock [Line Items]
Total Redemptions, Number of Units	11,838,040
Total Repurchases, Number of Shares	3,046,857
Total Redemptions and Repurchases, Number of Units and Shares	14,884,897
Total Redemptions, Average Price Paid per Unit	$ 4.97
Total Repurchases, Average Price Paid per Share	$ 5.09
Total Redemptions and Repurchases, Average Price Paid per Unit or Share	$ 4.99
Total redemption, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan
Total repurchases, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan	(121,753,000)	(110,090,000)
Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan	212,062,601
January 1, 2013-March 31, 2013 [Member]
Class of Stock [Line Items]
Total Redemptions, Number of Units	5,193,534
Total Repurchases, Number of Shares
Total Redemptions, Average Price Paid per Unit	$ 4.16
Total Repurchases, Average Price Paid per Share
Total redemption, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan
Total repurchases, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan
April 1, 2013-June 30, 2013 [Member]
Class of Stock [Line Items]
Total Redemptions, Number of Units	2,658,463
Total Repurchases, Number of Shares	33,478
Total Redemptions, Average Price Paid per Unit	$ 5.49
Total Repurchases, Average Price Paid per Share	$ 5.61
Total redemption, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan
Total repurchases, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan
July 1, 2013- September 30, 2013 [Member]
Class of Stock [Line Items]
Total Redemptions, Number of Units	452,115
Total Repurchases, Number of Shares	966,244
Total Redemptions, Average Price Paid per Unit	$ 5.70
Total Repurchases, Average Price Paid per Share	$ 5.77
Total redemption, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan
Total repurchases, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan
October 1, 2013-December 31, 2013 [Member]
Class of Stock [Line Items]
Total Redemptions, Number of Units	3,533,928
Total Repurchases, Number of Shares	2,047,135
Total Redemptions, Average Price Paid per Unit	$ 5.67
Total Repurchases, Average Price Paid per Share	$ 4.76
Total redemption, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan
Total repurchases, Approximate Dollar Value of Units and Shares That May Yet Be Redeemed/Purchased Under the Plan