Stockholders' Equity of the Company (At-the-Market Equity Offering Program and Perpetual Preferred Stock)(Details) - USD ($)	1 Months Ended	9 Months Ended		12 Months Ended
	Aug. 31, 2012	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Mar. 31, 2015	Feb. 28, 2015
Stockholders' Equity
Proceeds from sale of common stock		$ 0	$ 124,794,000
Common stock, shares authorized (in shares)		160,000,000		160,000,000
New At-the-Market Equity offering programs
Stockholders' Equity
Aggregate value of the shares of common stock to be sold (up to)					$ 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 (in shares)		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
Perpetual Preferred stock, annual dividends per share (in dollars per share)		$ 1.625
Contributions of net proceeds from sale of preferred stock	$ 221,900,000
Dividend Reinvestment and Stock Purchase Plan (DRIP)
Stockholders' Equity
Common stock, shares authorized (in shares)						3,500,000