Shareholders' Equity (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2012	Dec. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share	$ 25.00								$ 25.00
Shareholders' Equity, Common Stock [Roll Forward]
Balance as of beginning of period			54,310,484				54,268,915		54,310,484	54,268,915
Common stock issued under DRIP									16,753	515,257
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	54,739,111			54,310,484					54,739,111	54,310,484
Dividend reinvestment plan, shares authorized	3,000,000								3,000,000
Stock issued during period, remaining shares available, dividend reinvestment plan	2,450,145								2,450,145
Dividends declared per common share	$ 0.25	$ 0.25	$ 0.25	$ 0.27	$ 0.27	$ 0.29	$ 0.29	$ 0.25
Stock Repurchase Program, Authorized Amount	$ 50,000,000								$ 50,000,000
Stock Repurchased During Period, Shares									0	(872,447)
At the market progam [Member]
Shareholders' Equity, Common Stock [Roll Forward]
Common stock issued under ATM program									0	180,986
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage									8.50%
Preferred Stock, Shares Issued	2,300,000			2,300,000					2,300,000	2,300,000
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage									7.625%
Preferred Stock, Shares Issued	2,250,000			2,250,000					2,250,000	2,250,000
Common Stock [Member] | At the market progam [Member]
Shareholders' Equity, Common Stock [Roll Forward]
ATM program, authorized shares remaining	7,416,520								7,416,520