Shareholders' Equity (Preferred and Warrant) (Details) - USD ($)		1 Months Ended
	Aug. 01, 2018	Jul. 31, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cumulative Perpetual Preferred Stock
Warrant outstanding (in shares)			2,200,000.0	2,200,000.0	2,200,000.0
Warrant, exercise price (per share)			$ 65.52	$ 65.52	$ 65.52
Series C Preferred Stock
Cumulative Perpetual Preferred Stock
Preferred stock, shares issued		5,200,000.0
Preferred stock, liquidation value		$ 25.00
Preferred stock, dividend rate, percentage		7.875%
Preferred stock, redemption price per share			$ 25.00
Series C Preferred Stock | Forecast | LIBOR
Cumulative Perpetual Preferred Stock
Preferred stock, dividend rate, percentage	6.39%