Stockholders' Equity of the Company (Details 2) (USD $)	1 Months Ended	3 Months Ended								12 Months Ended					1 Months Ended	12 Months Ended		1 Months Ended		6 Months Ended		0 Months Ended	1 Months Ended	12 Months Ended
	Nov. 30, 2013	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 At-the-market equity offering programs	Jul. 31, 2011 At-the-market equity offering programs	Aug. 31, 2012 Series I Preferred Stock	Dec. 31, 2013 Series I Preferred Stock	Dec. 31, 2012 Series I Preferred Stock	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	Jul. 14, 2012 Series D Preferred Stock	Jul. 31, 2012 Series D Preferred Stock	Dec. 31, 2013 Dividend Reinvestment and Stock Purchase Plan (DRIP)	Dec. 31, 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
Issuance of common stock (shares)	2,600,000												462,276											761	1,300,000
Aggregate gross proceeds from shares sold													42,500,000
Net proceeds from sale of common stock										290,699,000	201,279,000	516,168,000	41,800,000											66,600	99,600,000
Aggregate value of shares available for issuance													2,800,000
Preferred stock, shares outstanding (shares)																9,200,000	9,200,000	7,700,000	11,700,000	7,700,000	7,700,000
Perpetual Preferred Units (as a percent)																6.50%		7.625%		7.625%			7.875%
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)																$ 1.625		$ 0.3495	$ 0.3707	$ 1.90625		$ 0.4922	$ 1.96875
Net proceeds from sale of preferred stock										0	221,965,000	0			221,900,000
Common stock, shares authorized (shares)		160,000,000				160,000,000				160,000,000	160,000,000															3,500,000
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		$ 0	$ 0	$ 12,160,000	$ 0	$ 0	$ 10,010,000	$ 0	$ 0	$ 12,160,000	$ 10,010,000	$ 0						$ 12,200,000	$ 6,300,000				$ 3,700,000