Stockholders' Equity of the Company - At-the-Market Equity Offering Program and Perpetual Preferred Stock (Details) - Series I Preferred Stock - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Aug. 31, 2012	Dec. 31, 2020	Dec. 31, 2019
Stockholders' Equity
Preferred stock, shares outstanding (in shares)	9,200,000	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	$ 25.00
Perpetual Preferred stock, annual dividends per share (in dollars per share)		$ 1.625
Contributions of net proceeds from sale of preferred stock	$ 221.9