Equity (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2012	Mar. 31, 2018	Dec. 31, 2017
Common Stock Repurchase
Repurchase of common stock authorized by Board of Directors		$ 100.0
Number of years for repurchase of common stock		5 years
Common stock repurchased, value	$ 10.2
Remaining authorized stock repurchase amount	$ 19.2
Temporary Equity, Par or Stated Value Per Share			$ 0.01	$ 0.01
Common Stock
Common Stock Repurchase
Common stock repurchased (in shares)	297,367
Series A-1 Preferred Stock
Common Stock Repurchase
Dividend rate (as a percent)	6.50%
Preferred stock dividends paid	$ 1.6
Series A-2 Preferred Stock
Common Stock Repurchase
Dividend rate (as a percent)	6.50%
Preferred stock dividends paid	$ 0.9
Dutch Auction [Member]
Common Stock Repurchase
Weighted average price per share (in dollars per share)	$ 34.24