Shareholders' Equity (Details) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
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		54,739,111	54,310,484
Common stock issued under DRIP		9,688	6,543
Common stock issued or redeemed under stock and incentive Plans		263,829.000	471,210
Common stock forfeited for tax withholding on share-based compensation		(67,296)	(59,150)
Stock Repurchased During Period, Shares		(860,721)	0
Balance as of end of period	54,084,611	54,084,611
Dividend reinvestment plan, shares authorized	3,000,000	3,000,000
Stock issued during period, remaining shares available, dividend reinvestment plan	2,440,457	2,440,457
Dividends declared per common share	$ 0.24
Stock Repurchase Program, Authorized Amount	$ 50,000,000	$ 50,000,000
Stock Repurchase Program Expiration Date		Dec. 31, 2016
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 43,311,000	$ 43,311,000
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		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
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