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, 2018	Dec. 31, 2017	Mar. 31, 2015
Stockholders' Equity
Preferred stock, shares outstanding (in shares)		9,200,000	9,200,000
Perpetual Preferred stock, liquidation preference (in dollars per share)		$ 25.00	$ 25.00
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 (in shares)	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