Equity (Details 2) (USD $)	6 Months Ended					6 Months Ended				6 Months Ended		6 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 Second Amended and Restated 2005 Stock Option and Incentive Plan	Jun. 30, 2012 Full-value awards fungibleunits	Jun. 30, 2012 Stock options, stock appreciation rights and other awards fungibleunits	Jun. 30, 2012 All other awards fungibleunits	Jun. 30, 2012 Series C Preferred Stock	Dec. 31, 2011 Series C Preferred Stock	Jun. 30, 2012 Series D Preferred Stock	Dec. 31, 2011 Series D Preferred Stock	Jun. 30, 2012 At-the-market equity offering programs	Jul. 31, 2011 At-the-market equity offering programs	Jun. 30, 2012 Dividend Reinvestment and Stock Purchase Plan (DRIP)	Jun. 30, 2011 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												2,600,000		1,300,000	87
Aggregate gross proceeds from shares sold												204,600,000
Net proceeds from sale of common stock	201,307,000	419,463,000										201,600,000		99,500,000	6,500
Aggregate value of shares available for issuance												$ 45,400,000
Perpetual Preferred stock, shares outstanding								11,700,000	11,700,000	4,000,000	4,000,000
Perpetual Preferred Units (as a percent)								7.625%		7.875%
Perpetual Preferred stock, liquidation preference (in dollars per share)								$ 25.00	$ 25.00	$ 25.00	$ 25.00
Perpetual Preferred stock, annual dividends per share (in dollars per share)								$ 1.90625		$ 1.96875
Common stock, shares authorized	160,000,000		160,000,000													3,500,000
Maximum fungible units that may be granted (in shares)						10,730,000
Fungible units per share					1.65	0.79	1.0
Award expiration period (in years)						P5Y	P10Y
Shares that may be issued if equal to fungible units				10,730,000
Fungible units available for issuance (in shares)				4,100,000
Fungible units available for issuance if issued as five year stock options (in shares)				5,200,000
Preferred stock, par value (in dollars per share)	$ 0.01							$ 0.01	$ 0.01	$ 0.01	$ 0.01