Stockholders' Equity of the Company - Perpetual Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	9 Months Ended
	Aug. 31, 2012	Sep. 30, 2018	Dec. 31, 2017	Feb. 28, 2015
Stockholders' Equity
Common stock, shares authorized (in shares)		160,000,000	160,000,000
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	$ 221.9
Dividend Reinvestment and Stock Purchase Plan (DRIP)
Stockholders' Equity
Common stock, shares authorized (in shares)				3,500,000