Preferred Stock - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended		12 Months Ended					12 Months Ended					1 Months Ended		12 Months Ended
	Oct. 24, 2011	Sep. 14, 2012	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Mar. 04, 2011	Dec. 31, 2012 Cumulative Series C Six Point Seven Five Percent Redeemable Perpetual Preferred Stock [Member]	Sep. 14, 2012 Cumulative Series C Six Point Seven Five Percent Redeemable Perpetual Preferred Stock [Member]	Dec. 31, 2011 Cumulative Series C Six Point Seven Five Percent Redeemable Perpetual Preferred Stock [Member]	Dec. 31, 2012 8.034% Series A Cumulative Redeemable Perpetual Preferred Stock	Dec. 31, 2011 8.034% Series A Cumulative Redeemable Perpetual Preferred Stock	Dec. 23, 2011 Series B Subordinated Non-Voting Cumulative Redeemable Preferred Stock	Oct. 24, 2011 Series B Subordinated Non-Voting Cumulative Redeemable Preferred Stock	Dec. 31, 2012 Series B Subordinated Non-Voting Cumulative Redeemable Preferred Stock	Dec. 31, 2011 Series B Subordinated Non-Voting Cumulative Redeemable Preferred Stock	Dec. 31, 2010 Series B Subordinated Non-Voting Cumulative Redeemable Preferred Stock
Auction Market Preferred Securities, Stock Series [Line Items]
Preferred Stock, shares authorized			10,000,000	10,000,000
Preferred Stock, par value per share			$ 0.01	$ 0.01	$ 0.01				$ 0.01		$ 0.01	$ 0.01	$ 0.01
Cumulative Redeemable Preferred Stock, shares issued			0	0	8,000,000			8,000,000
Stock issued for acquisition properties	3,162,069													286,207	1,453,793	0	1,740,000	0
Cumulative Redeemable Preferred Stock, par value per share			$ 0.01	$ 0.01	$ 0.01			$ 0.01									$ 0.01
Cumulative Redeemable Preferred Stock, liquidation preference per share								$ 25.00
Cumulative Redeemable Preferred Stock, dividend rate									6.75%
Cumulative Redeemable Preferred Stock, conversion price per share								$ 24.75
Cumulative Redeemable Preferred Stock, redemption price per share								$ 25.00
Perpetual Preferred OP Units conversion				$ 0	$ 200,000,000	$ 0
Common Stock, Par or Stated Value Per Share			$ 0.01	$ 0.01	$ 0.01
At The Market Offering Value							$ 125,000,000
Preferred Stock, Shares Authorized			9,945,542	9,945,542	2,000,000				54,461	54,458	0	0	0
Preferred Stock, Liquidation Preference Per Share										$ 2,500.00
Peferred Stock, Shares Exchanged		5,445,765
Depositary Shares, Issued		5,445,765
Depositary Share, Liquidation Preference Per Share		$ 25.00
Preferred Stock, Dividend Rate, Per-Dollar-Amount		$ 0.3849625	$ 0.0948460