Stockholders' Equity (Details 2) (USD $)	3 Months Ended	6 Months Ended				6 Months Ended			1 Months Ended		6 Months Ended			1 Months Ended	6 Months Ended	1 Months Ended	6 Months Ended		6 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Full-value awards fungibleunits	Jun. 30, 2013 Stock options, stock appreciation rights and other awards that do no deliver full value sqft	Jun. 30, 2013 All other awards fungibleunits	Jun. 30, 2013 Third Amended and Restated 2005 Stock Option and Incentive Plan	Jun. 30, 2013 Series C Preferred Stock	Sep. 30, 2012 Series C Preferred Stock	Jun. 30, 2013 Series C Preferred Stock	Dec. 31, 2012 Series C Preferred Stock	Aug. 31, 2012 Series C Preferred Stock	Jul. 31, 2012 Series D Preferred Stock	Jun. 30, 2013 Series D Preferred Stock	Aug. 31, 2012 Series I Preferred Stock	Jun. 30, 2013 Series I Preferred Stock	Dec. 31, 2012 Series I Preferred Stock	Jun. 30, 2013 At-the-market equity offering programs	Jul. 31, 2011 At-the-market equity offering programs	Jun. 30, 2013 Dividend Reinvestment and Stock Purchase Plan (DRIP)	Jun. 30, 2012 Dividend Reinvestment and Stock Purchase Plan (DRIP)	Mar. 31, 2012 Dividend Reinvestment and Stock Purchase Plan (DRIP)
Stockholders' Equity
Aggregate value of the shares of common stock to be sold																				$ 250,000,000
Common stock, shares issued																			95,000		312	1,300,000
Aggregate gross proceeds from shares sold																			8,700,000
Net proceeds from sale of common stock/preferred stock		8,478,000	201,307,000																8,500,000		26,000	99,500,000
Net proceeds from sale of preferred stock																221,900,000
Aggregate value of shares available for issuance																			36,700,000
Perpetual Preferred stock, shares outstanding												7,700,000	11,700,000				9,200,000	9,200,000
Perpetual Preferred Units (as a percent)									7.625%					7.875%			6.50%
Perpetual Preferred stock, liquidation preference (in dollars per share)									$ 25.00		$ 25.00	$ 25.00					$ 25.00	$ 25.00
Perpetual Preferred stock, annual dividends per share (in dollars per share)											$ 1.90625				$ 1.96875		$ 1.625
Amount of accumulated and unpaid dividends (in dollars per share)									$ 0.3495	$ 0.3707				$ 0.4922
Redemption price of preferred stock (in dollars per share)									$ 25.00	$ 25.00	$ 25.00			$ 25.00
Preferred stock redeemed (in shares)									7,700,000	4,000,000				4,000,000
Preferred stock redemption costs	$ 12,160,000	$ 12,160,000							$ 12,200,000	$ 6,300,000				$ 3,700,000
Common stock, shares authorized	160,000,000	160,000,000		160,000,000																			3,500,000
Maximum fungible units that may be granted (in shares)								17,130,000
Fungible units available for issuance (in shares)								5,800,000
Fungible units per share					2.76	0.77	1.0
Award expiration period						5 years	10 years
Shares that may be issued if equal to fungible units								17,130,000
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01							$ 0.01		$ 0.01	$ 0.01					$ 0.01	$ 0.01