Stockholders' Equity of the Company (Details 2) (USD $)	6 Months Ended		6 Months Ended		1 Months Ended	6 Months Ended		6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 At-the-market equity offering programs	Jul. 31, 2011 At-the-market equity offering programs	Jun. 30, 2014 New at-the-market equity offering program	Jun. 30, 2014 Series I Preferred Stock	Dec. 31, 2013 Series I Preferred Stock	Jun. 30, 2014 Dividend Reinvestment and Stock Purchase Plan (DRIP)	Jun. 30, 2012 Dividend Reinvestment and Stock Purchase Plan (DRIP)
Stockholders' Equity
Aggregate value of the shares of common stock to be sold				$ 250,000,000.0	$ 300,000,000.0
Issuance of common stock (shares)			25,659		55,765			272
Proceeds from shares sold			2,800,000		6,100,000
Aggregate value of shares available for issuance					293,800,000
Preferred stock, shares outstanding (shares)						9,200,000	9,200,000
Dividend rate Perpetual Preferred Units (as a percent)						6.50%
Perpetual Preferred stock, liquidation preference (in dollars per share)						$ 25.00	$ 25.00
Perpetual Preferred stock, annual dividends per share (in dollars per share)						$ 1.625
Common stock, shares authorized (shares)	160,000,000	160,000,000							3,500,000
DRIP proceeds	$ 26,000							$ 26,000