Stockholders' Equity of the Company (At-the-Market Equity Offering Program and Perpetual Preferred Stock)(Details) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended
	Aug. 31, 2012	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Jul. 31, 2011
Stockholders' Equity
Contributions - net proceeds from common stock offering (in shares)					1,626,999
Proceeds from sale of common stock			$ 0	$ 124,761,000	$ 185,321,000
Preferred stock, shares outstanding			9,200,000	9,200,000
Perpetual Preferred stock, liquidation preference (in dollars per share)			$ 25.00	$ 25.00
At-the-Market Equity Offering Programs
Stockholders' Equity
Aggregate value of the shares of common stock to be sold							$ 250,000,000.0
Contributions - net proceeds from common stock offering (in shares)				25,659
Proceeds from sale of common stock		$ 113,400,000		$ 2,800,000
New At-the-Market Equity offering programs
Stockholders' Equity
Aggregate value of the shares of common stock to be sold						$ 300,000,000.0
Contributions - net proceeds from common stock offering (in shares)				895,956	1,626,999
Proceeds from sale of common stock					$ 182,900,000
2015 New At-the-Market Equity Offering Program [Member]
Stockholders' Equity
Aggregate value of the shares of common stock to be sold		$ 300,000,000.0
Contributions - net proceeds from common stock offering (in shares)				91,180
Proceeds from sale of common stock				$ 12,000,000
Series I Preferred Stock
Stockholders' Equity
Preferred stock, shares outstanding	9,200,000		9,200,000
Dividend rate preferred units (as a percent)	6.50%		6.50%
Perpetual Preferred stock, liquidation preference (in dollars per share)			$ 25.00
Perpetual Preferred stock, annual dividends per share (in dollars per share)			$ 1.625
Contributions of net proceeds from sale of preferred stock	$ 0