Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended						6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 At the market progam [Member]	Jun. 30, 2013 At the market progam [Member]	Jun. 30, 2014 Series A Preferred Stock [Member]	Dec. 31, 2013 Series A Preferred Stock [Member]	Jun. 30, 2014 Series B Preferred Stock [Member]	Dec. 31, 2013 Series B Preferred Stock [Member]	Jun. 30, 2014 Common Stock [Member] At the market progam [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage							8.50%		7.625%
Preferred Stock, Liquidation Preference Per Share	$ 25.00		$ 25.00
Preferred Stock, Shares Issued							2,300,000	2,300,000	2,250,000	2,250,000
Shareholders' Equity, Common Stock [Roll Forward]
Balance as of beginning of period			54,310,484	54,268,915
Common stock issued under DRIP			6,543	505,718
Common stock issued under ATM program					0	180,986
Common stock issued or redeemed under stock and incentive Plans			471,210	270,158
Common stock forfeited for tax withholding on share-based compensation			(59,150)	(52,385)
Balance as of end of period	54,729,087	55,173,392	54,729,087	55,173,392
Dividend reinvestment plan, shares authorized	3,000,000		3,000,000
Stock issued during period, remaining shares available, dividend reinvestment plan	2,460,355		2,460,355
Dividends declared per common share	$ 0.25	$ 0.29	$ 0.50	$ 0.58
ATM program, authorized shares remaining											7,416,520
Stock Repurchase Program, Authorized Amount			$ 50,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount			$ 42