Share and Unit Information - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Mar. 02, 2012	Dec. 31, 2011	Aug. 26, 2010	Nov. 05, 2009	Jul. 03, 2008
Stockholders Equity Note [Line Items]
Common stock, shares outstanding	41,925,288	37,823,972	41,925,288	37,823,972		38,959,338
Operating partnership unit outstanding	1,774,547	1,942,275	1,774,547	1,942,275
Total common shares and operating partnership units outstanding	43,699,835	39,766,247	43,699,835	39,766,247
Common stock shares, outstanding option	0		0
Agreement to sell common stock with financial service companies, shares							6,000,000	4,000,000	1,350,000
Proceeds from Issuance of common stock			$ 173,960,000	$ 170,515,000
Shares Sold On Underwritten Public Offering					1,955,000
Common stock issued, Dividend and Distribution Reinvestment and Share Purchase Program	122	100	451	495,487
Proceeds from Issuance of common stock, Dividend and Distribution Reinvestment and Share Purchase Program	8,000	7,039	30,000	30,000,000
Common stock acquired to satisfy tax withholding obligation that arose upon vesting of restricted stock granted			15,565	25,082
At The Market ATM Programs [Member]
Stockholders Equity Note [Line Items]
Common stock, shares issued	600,175	656,500	812,911	2,178,973
proceeds from issuance of common stock, net of applicable fees		44,200,000	53,700,000	139,500,000
Proceeds from Issuance of common stock		44,900,000	54,500,000	141,700,000
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