Stockholders' Equity of the Company (Details 2) - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended
	Aug. 31, 2012	Mar. 31, 2015	Sep. 30, 2015	Sep. 30, 2014	Feb. 28, 2015	Dec. 31, 2014	Jun. 30, 2014
Stockholders' Equity
Proceeds from sale of common stock			$ 124,794,000	$ 42,698,000
Common stock, shares authorized			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							$ 300,000,000.0
Contributions - net proceeds from common stock offering (in shares)		895,956
Proceeds from sale of common stock		$ 113,400,000
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)			91,180
Proceeds from sale of common stock			$ 12,000,000
Aggregate value of shares available for issuance			$ 288,000,000
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%
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
Contributions - net proceeds from common stock offering (in shares)			775,529
Proceeds from sale of common stock			$ 99,500,000
Common stock, shares authorized					3,500,000