Shareholders' Equity (Details) (USD $)	3 Months Ended		9 Months Ended						9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 At the market progam [Member]	Sep. 30, 2013 At the market progam [Member]	Sep. 30, 2014 Series A Preferred Stock [Member]	Dec. 31, 2013 Series A Preferred Stock [Member]	Sep. 30, 2014 Series B Preferred Stock [Member]	Dec. 31, 2013 Series B Preferred Stock [Member]	Sep. 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			12,459	509,831
Common stock issued under ATM program					0	180,986
Common stock issued or redeemed under stock and incentive Plans			471,210.00	270,158
Common stock forfeited for tax withholding on share-based compensation			(59,336)	(52,385)
Balance as of end of period	54,734,817	54,426,049	54,734,817	54,426,049
Dividend reinvestment plan, shares authorized	3,000,000		3,000,000
Stock issued during period, remaining shares available, dividend reinvestment plan	2,454,439		2,454,439
Dividends declared per common share	$ 0.25	$ 0.27	$ 0.75	$ 0.85
ATM program, authorized shares remaining											7,416,520
Stock Repurchase Program, Authorized Amount	$ 50,000,000		$ 50,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 42,145,000		$ 42,145,000
Stock Repurchased During Period, Shares			0	(751,456)