Shareholders' Equity - Additional Information (Detail) (USD $)	1 Months Ended			9 Months Ended		36 Months Ended
	Jan. 31, 2012	Jan. 31, 2009	Oct. 31, 2006	Sep. 30, 2012	Sep. 30, 2011	Jan. 31, 2012	Dec. 31, 2011	Oct. 24, 2006
Class of Stock [Line Items]
Authorized aggregate number of shares				1,000,000,000
Common Shares of Beneficial Interest, shares authorized				990,000,000			990,000,000
Common shares, par value				$ 0.01			$ 0.01
Preferred shares under beneficial interest				10,000,000
Capital commitment funded		$ 3,000,000,000	$ 2,000,000,000
Percentage of Common Shares Offered in Beneficial Interest at Specified Per Share Amount		90.00%						90.00%
Offer Price of 90% of the common shares in beneficial interest		$ 10.00						$ 10.00
Percentage of shares offered pursuant to dividend reinvestment plan		10.00%						10.00%
Condition 1 for shares offered under dividend reinvestment plan		$ 9.50						$ 9.50
Condition 2 for shares offered under dividend reinvestment plan as a percentage of fair market value of common share		95.00%						95.00%
Capital commitment funded	190,672,251	60,808,967	60,808,967			190,672,251
Gross proceeds from public offering	1,901,137,211		607,345,702	171,086,000	432,455,000	1,901,137,211
Repurchased of common shares				3,841,599	2,786,617
Proceeds from Share Redemption Program				$ 35,888,000	$ 25,477,000