Summary of Income Tax Status of Dividends Per Share Paid (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011
Dividends [Line Items]
Ordinary income									$ 0.56	$ 0.22
Return of capital									$ 0.28	$ 0.58
Total	$ 0.21	$ 0.21	$ 0.21	$ 0.21	$ 0.21	$ 0.21	$ 0.21	$ 0.17	$ 0.84	$ 0.80
Ordinary income									66.70%	27.50%
Return of capital									33.30%	72.50%
Total									100.00%	100.00%