Private Placement Offering of Preferred Stock (Details) - Private Placement - Preferred Stock - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Private Placement Offering of Preferred Stock
Issuance of stock, net of offering expenses (in shares)	10,000		31,625	10,000
Share price per share	$ 1,000		$ 1,000.00	$ 1,000
Gross proceeds from offering	$ 10.0		$ 31.6
Net proceeds after offering expenses			$ 31.6
Dividend yield		8.00%