Stockholders' Equity of the Company - At-the-Market Equity Offering Program and Perpetual Preferred Stock (Details) - USD ($)	1 Months Ended	12 Months Ended
	Aug. 31, 2012	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Jun. 30, 2014	Jul. 31, 2011
Stockholders' Equity
Proceeds from sale of common stock		$ 0	$ 0	$ 124,761,000
Preferred stock, shares outstanding		9,200,000	9,200,000
Perpetual Preferred stock, liquidation preference (in dollars per share)		$ 25.00	$ 25.00
2011 ATM Equity Offering Program
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				$ 2,800,000
2014 ATM Equity Offering Program
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
2015 ATM Equity Offering Program
Stockholders' Equity
Aggregate value of the shares of common stock to be sold					$ 300,000,000.0
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	$ 221,900,000