Shareholders' Equity (Summary of Preferred Stock) (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Preferred stock, shares authorized	12,500,000
Preferred stock	$ 159,541,000	$ 207,508,000
Series B [Member]
Class Of Stock [Line Items]
Par value	$ 0.01
Liquidation value	$ 25.00
Preferred stock, shares authorized	1,450,000	3,450,000
Shares issued	1,450,000	3,450,000
Shares outstanding	1,450,000	3,450,000
Preferred stock	$ 34,767,000	$ 82,734,000
Series C Preferred Stock [Member]
Class Of Stock [Line Items]
Par value	$ 0.01
Liquidation value	$ 25.00
Preferred stock, shares authorized	6,450,000	6,450,000
Shares issued	5,000,000	5,000,000
Shares outstanding	5,000,000	5,000,000
Preferred stock	$ 124,774,000	$ 124,774,000