Equity (Details 2) (USD $)	3 Months Ended	12 Months Ended				12 Months Ended		1 Months Ended	12 Months Ended		0 Months Ended	1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended			12 Months Ended
	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Second Amended and Restated 2005 Stock Option and Incentive Plan	Dec. 31, 2012 Full-value awards fungibleunits	Dec. 31, 2012 Stock options, stock appreciation rights and other awards fungibleunits	Dec. 31, 2012 All other awards fungibleunits	Sep. 30, 2012 Series C Preferred Stock	Dec. 31, 2012 Series C Preferred Stock	Dec. 31, 2011 Series C Preferred Stock	Jul. 14, 2012 Series D Preferred Stock	Jul. 31, 2012 Series D Preferred Stock	Dec. 31, 2012 Series D Preferred Stock	Dec. 31, 2011 Series D Preferred Stock	Aug. 31, 2012 Series I Preferred Stock	Dec. 31, 2012 Series I Preferred Stock	Dec. 31, 2012 At-the-market equity offering programs	Jul. 31, 2011 At-the-market equity offering programs	Dec. 31, 2012 Dividend Reinvestment and Stock Purchase Plan (DRIP)	Dec. 31, 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	473
Aggregate gross proceeds from shares sold																	204,600,000
Net proceeds from sale of common stock		201,279,000	516,168,000												222,200,000		201,300,000		99,600,000	34,000
Aggregate value of shares available for issuance																	45,400,000
Perpetual Preferred stock, shares outstanding									7,700,000	11,700,000			0	4,000,000		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)								$ 0.3707	$ 1.90625		$ 0.4922		$ 1.96875			$ 1.625
Preferred stock redeemed (in shares)								4,000,000				4,000,000
Preferred stock Redemption (in dollars per share)								$ 25.00				$ 25.00
Preferred stock redemption costs	$ 10,010,000	$ 10,010,000						$ 6,300,000				$ 3,700,000
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						5 years	10 years
Shares that may be issued if equal to fungible units				10,730,000
Preferred stock, par value (in dollars per share)		$ 0.01							$ 0.01	$ 0.01			$ 0.01	$ 0.01		$ 0.01