Stockholders' Equity (Details 2) (USD $)	3 Months Ended					3 Months Ended		1 Months Ended	3 Months Ended			0 Months Ended	1 Months Ended	3 Months Ended	1 Months Ended	3 Months Ended				3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Second Amended and Restated 2005 Stock Option and Incentive Plan	Mar. 31, 2013 Full-value awards fungibleunits	Mar. 31, 2013 Stock options, stock appreciation rights and other awards fungibleunits	Mar. 31, 2013 All other awards fungibleunits	Sep. 30, 2012 Series C Preferred Stock	Mar. 31, 2013 Series C Preferred Stock	Dec. 31, 2012 Series C Preferred Stock	Aug. 31, 2012 Series C Preferred Stock	Jul. 14, 2012 Series D Preferred Stock	Jul. 31, 2012 Series D Preferred Stock	Mar. 31, 2013 Series D Preferred Stock	Aug. 31, 2012 Series I Preferred Stock	Mar. 31, 2013 Series I Preferred Stock	Dec. 31, 2012 Series I Preferred Stock	Mar. 31, 2013 At-the-market equity offering programs	Jul. 31, 2011 At-the-market equity offering programs	Mar. 31, 2013 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
Aggregate value of shares available for issuance																		45,400,000
Perpetual Preferred stock, shares outstanding									7,700,000	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
Perpetual Preferred stock, annual dividends per share (in dollars per share)								$ 0.3707	$ 1.90625			$ 0.4922		$ 1.96875		$ 1.625
Net proceeds from sale of common stock/preferred stock	(57,000)	122,953,000													221,900,000					13,000	99,500,000
Preferred stock redeemed (in shares)													4,000,000
Preferred stock Redemption (in dollars per share)													$ 25.00
Preferred stock redemption costs								$ 6,300,000					$ 3,700,000
Common stock, shares authorized	160,000,000		160,000,000																		3,500,000
Common stock, shares issued																				170	1,300,000
Maximum fungible units that may be granted (in shares)				10,730,000
Fungible units available for issuance (in shares)				0
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