Stockholders' Equity of the REIT - Equity Securities Distributions Paid or Payable Per Unit (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Class of Stock [Line Items]
Ordinary income	$ 0.38	$ 0.07
Qualified dividend	$ 0.20	$ 0.01
Capital gains	$ 0.54	$ 0.84	$ 1.25
Return of capital		$ 0.14
Total dividend	$ 1.12	$ 1.06	$ 1.25
Series L [Member]
Class of Stock [Line Items]
Ordinary income	$ 0.55	$ 0.15
Qualified dividend	$ 0.28
Capital gains	$ 0.80	$ 1.07
Total dividend	$ 1.63	$ 1.22
Series M [Member]
Class of Stock [Line Items]
Ordinary income	$ 0.57	$ 0.15
Qualified dividend	$ 0.30
Capital gains	$ 0.82	$ 1.11
Total dividend	$ 1.69	$ 1.26
Series O [Member]
Class of Stock [Line Items]
Ordinary income	$ 0.59	$ 0.16
Qualified dividend	$ 0.31
Capital gains	$ 0.85	$ 1.15
Total dividend	$ 1.75	$ 1.31
Series P [Member]
Class of Stock [Line Items]
Ordinary income	$ 0.58	$ 0.15
Qualified dividend	$ 0.30
Capital gains	$ 0.83	$ 1.13
Total dividend	$ 1.71	$ 1.28
Series Q [Member]
Class of Stock [Line Items]
Ordinary income	$ 1.44	$ 0.38
Qualified dividend	$ 0.75	$ 0.04
Capital gains	$ 2.08	$ 3.85	$ 4.27
Total dividend	$ 4.27	$ 4.27	$ 4.27
Series R [Member]
Class of Stock [Line Items]
Ordinary income	$ 0.57	$ 0.15
Qualified dividend	$ 0.30	$ 0.02
Capital gains	$ 0.82	$ 1.52	$ 1.69
Total dividend	$ 1.69	$ 1.69	$ 1.69
Series S [Member]
Class of Stock [Line Items]
Ordinary income	$ 0.57	$ 0.15
Qualified dividend	$ 0.30	$ 0.02
Capital gains	$ 0.82	$ 1.52	$ 1.69
Total dividend	$ 1.69	$ 1.69	$ 1.69