Stockholders' Equity (Details) (USD $)	0 Months Ended			6 Months Ended				6 Months Ended				0 Months Ended	60 Months Ended					6 Months Ended
	Apr. 30, 2013	Mar. 27, 2013	Jan. 29, 2013	Jun. 30, 2013	Jun. 18, 2013	Dec. 31, 2012	Jun. 30, 2013 Atm Program [Member]	Jun. 30, 2013 Issued in conjunction with debt exchange for 3.75% exchangeable senior notes [Member]	Jun. 30, 2013 Quarter 4 2012 [Member]	Jun. 30, 2013 Quarter 1 2013 [Member]	Jun. 30, 2013 Quarter 2 2013 [Member]	May 15, 2013 Series B Cumulative Redeemable Preferred Stock [Member]	Mar. 26, 2018 Series B Cumulative Redeemable Preferred Stock [Member]	Mar. 27, 2018 Series B Cumulative Redeemable Preferred Stock [Member]	Jun. 30, 2013 Series B Cumulative Redeemable Preferred Stock [Member]	Jun. 18, 2013 Series B Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2012 Series B Cumulative Redeemable Preferred Stock [Member]	Jun. 30, 2013 Series B Cumulative Redeemable Preferred Stock [Member] Quarter 2 2013 [Member]	Jun. 30, 2013 Series B Cumulative Redeemable Preferred Stock [Member] Quarter 3 2013 [Member]	Mar. 27, 2018 Depository shares of Series B Cumulative Redeemable Preferred [Member]	Jun. 30, 2013 Depository shares of Series B Cumulative Redeemable Preferred [Member]
Class of Stock [Line Items]
Common stock, shares outstanding				152,384,740		151,601,209
At the market stock offering program, aggregate value of common stock							$ 600,000,000
At the market stock offering program remaining amount available for issuance							305,300,000
Shares of its Common Stock in connection with the redemption of an equal number of OP Units				333,997
Dividend payable date									Jan. 29, 2013	Apr. 30, 2013	Jul. 31, 2013							May 15, 2013	Aug. 15, 2013
Dividend paid on common stock (in dollars per share)	$ 0.65		$ 0.65									$ 17.8646
Dividend shareholder of record date									Dec. 31, 2012	Mar. 29, 2013	Jun. 28, 2013								Aug. 05, 2013
Dividends payable, date declared											Jun. 18, 2013								Jun. 18, 2013
Dividends Payable, Amount Per Share					$ 0.65											$ 32.8125
Series B, Shares Outstanding															80,000		0				8,000,000
Series B, Dividend Rate, Percentage													5.25%
Series B, Liquidation Preference Per Share														$ 2,500.00	$ 2,500.00		$ 0			$ 25.00	$ 25.00
Proceeds from Issuance of Series B		$ 193,900,000		$ 193,920,000
Ratio of depository shares to shares of Series B Preferred Stock															0.01
Shares issued in conjunction with exchange of 3.75% exchangeable senior notes								419,116
Coupon/Stated Rate								3.75%