Shareholders' Equity (Narrative) (Details) - USD ($)								12 Months Ended
	Jan. 16, 2019	Jan. 12, 2018	Dec. 15, 2017	Sep. 15, 2017	Aug. 24, 2017	Aug. 16, 2017	Aug. 01, 2016	Dec. 31, 2018	Dec. 31, 2017	Feb. 14, 2019	Dec. 18, 2018
Class Of Stock [Line Items]
Preferred stock, shares authorized								12,500,000
Net proceeds from sale of preferred stock									$ 120,432,000
Common stock repurchase, authorized											$ 30,000,000
Common stock repurchase, shares								772,000
Common stock repurchase, weighted average price per share								$ 3.02
Common stock, shares issued								90,436,000	91,317,000
Stock price as percentage of market value								100.00%
Remaining shares authorized under DRIP								2,836,000
Accrued preferred stock dividends								$ 1,200,000	$ 1,600,000
Subsequent Event [Member]
Class Of Stock [Line Items]
Additional number of common stock shares repurchased										2,050,000
Additional common stock repurchased, weighted average price per share										$ 3.34
Dividends payable, date declared	Jan. 16, 2019
Common stock, dividends declared	$ 0.05
Dividends payable, date to be paid	Feb. 20, 2019
Dividends payable, date of record	Feb. 08, 2019
Forward Sales Agreement [Member]
Class Of Stock [Line Items]
Common stock, shares issued							5,750,000
Settlement date of agreements							Aug. 01, 2017
Public offering, estimated net proceeds							$ 43,200,000
Series B [Member]
Class Of Stock [Line Items]
Dividend rate percentage								7.25%	7.25%
Redemption date								May 22, 2017
Redemption price per share		$ 25.00		$ 25.00		$ 25.00		$ 25.00
Preferred stock, shares authorized								1,450,000	3,450,000
Preferred stock redeemed, shares		2,000,000		3,000,000		1,500,000
Redemption price plus accrued and unpaid dividends amount		$ 50,000,000		$ 75,000,000		$ 37,500,000
Series B [Member] | Subsequent Event [Member]
Class Of Stock [Line Items]
Preferred stock dividends declared	$ 0.453125
Series C [Member]
Class Of Stock [Line Items]
Dividend rate percentage								6.50%	6.50%
Redemption date								Aug. 24, 2022
Redemption price per share								$ 25.00
Preferred stock, shares authorized								6,450,000	6,450,000
Common stock sales, net of issuance expenses, shares			2,000,000		3,000,000
Shares Issued, price per share			$ 25.00		$ 25.00
Net proceeds from sale of preferred stock			$ 48,100,000		$ 72,300,000
Series C [Member] | Subsequent Event [Member]
Class Of Stock [Line Items]
Preferred stock dividends declared	$ 0.406250