Share and Unit Information - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 02, 2012	Aug. 26, 2010
Stockholders Equity Note [Line Items]
Common stock, shares outstanding	42,683,322	40,940,360	42,316,398
Total common shares and operating partnership units outstanding	44,390,982	42,876,068
Common stock shares, outstanding option	0	0
Agreement to sell common stock with financial service companies, shares	4,500,000				6,000,000
Proceeds from Issuance of common stock	$ 22,058,000	$ 120,135,000
Shares Sold On Underwritten Public Offering				1,955,000
Common stock issued, Dividend and Distribution Reinvestment and Share Purchase Program	141	120
Proceeds from Issuance of common stock, Dividend and Distribution Reinvestment and Share Purchase Program	10,000	8,000
Treasury Stock, Shares, Acquired	4,582	15,565
At Market A T M Programs [Member]
Stockholders Equity Note [Line Items]
Stock Issued During Period, Shares, New Issues	325,166
At The Market ATM Programs [Member]
Stockholders Equity Note [Line Items]
Stock Issued During Period, Shares, New Issues	4,174,834
proceeds from issuance of common stock, net of applicable fees	22,000,000
Proceeds from Issuance of common stock	22,300,000
Initial Public Offering [Member]
Stockholders Equity Note [Line Items]
proceeds from issuance of common stock, net of applicable fees	120,000,000
Proceeds from Issuance of common stock		$ 124,100,000
Noncontrolling Interest [Member]
Stockholders Equity Note [Line Items]
Limited Partners' Capital Account, Units Outstanding	1,707,660	1,935,708