Shareholders' Equity (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share	$ 25.00
Shareholders' Equity, Common Stock [Roll Forward]
Balance as of beginning of period	54,739,111	54,310,484
Common stock issued under DRIP	4,744	3,938
Common stock issued or redeemed under stock and incentive Plans	231,274.000	442,035
Common stock forfeited for tax withholding on share-based compensation	(65,913)	(59,150)
Stock Repurchased During Period, Shares	(16,140)	0
Balance as of end of period	54,893,076	54,697,307
Dividend reinvestment plan, shares authorized	3,000,000
Stock issued during period, remaining shares available, dividend reinvestment plan	2,445,401
Dividends declared per common share	$ 0.24
Stock Repurchase Program, Authorized Amount	$ 50,000,000
Stock Repurchase Program Expiration Date	Dec. 31, 2016
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 49,868,000
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
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
Common Stock [Member] | At the market progam [Member]
Shareholders' Equity, Common Stock [Roll Forward]
ATM program, authorized shares remaining	7,416,520