Stockholders' Equity of the Company (Details 2) (USD $)	1 Months Ended	3 Months Ended								12 Months Ended			1 Months Ended			6 Months Ended
	Nov. 30, 2013	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Aug. 31, 2012	Jun. 30, 2013	Sep. 30, 2012	Jun. 30, 2013	Jul. 31, 2011	Mar. 31, 2012
Stockholders' Equity
Net proceeds from sale of common stock										$ 185,321,000	$ 290,699,000	$ 201,279,000
Issuance of common stock (shares)	2,600,000
Net proceeds from sale of preferred stock										0	0	221,965,000
Preferred stock redemption costs		0	0	0	0	0	0	12,160,000	0	0	12,160,000	10,010,000
Common stock, shares authorized (shares)		160,000,000				160,000,000				160,000,000	160,000,000
At-the-market equity offering programs
Stockholders' Equity
Aggregate value of the shares of common stock to be sold																	250,000,000.0
Net proceeds from sale of common stock										2,800,000
Issuance of common stock (shares)										25,659
New At-the-Market Equity offering programs
Stockholders' Equity
Aggregate value of the shares of common stock to be sold				300,000,000.0
Net proceeds from sale of common stock										182,900,000
Issuance of common stock (shares)										1,626,999
Series I Preferred Stock
Stockholders' Equity
Preferred stock, shares outstanding (shares)		9,200,000				9,200,000				9,200,000	9,200,000
Dividend rate preferred units (as a percent)										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)										$ 1.625
Net proceeds from sale of preferred stock													221,900,000
Series C Preferred Stock
Stockholders' Equity
Preferred stock, shares outstanding (shares)								7,700,000					11,700,000	7,700,000		7,700,000
Dividend rate preferred units (as a percent)														7.625%		7.625%
Perpetual Preferred stock, liquidation preference (in dollars per share)								$ 25.00						$ 25.00		$ 25.00
Perpetual Preferred stock, annual dividends per share (in dollars per share)														$ 0.3495	$ 0.3707	$ 1.90625
Preferred stock redemption costs														12,200,000	6,300,000
Preferred stock redeemed (in shares)															4,000,000
Preferred stock Redemption (in dollars per share)															$ 25.00
Dividend Reinvestment and Stock Purchase Plan (DRIP)
Stockholders' Equity
Net proceeds from sale of common stock										$ 63,720	$ 66,600	$ 99,600,000
Issuance of common stock (shares)										608	761	1,300,000
Common stock, shares authorized (shares)																		3,500,000