Shareholders' Equity (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	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	54,739,111	54,310,484
Common stock issued under DRIP									22,607	16,753
Common stock issued or redeemed under stock and incentive Plans									263,829	471,210
Common stock forfeited for tax withholding on share-based compensation									(67,296)	(59,336)
Stock Repurchased During Period, Shares									(5,910,916)	0
Balance as of end of period	49,047,335				54,739,111				49,047,335	54,739,111
Dividend reinvestment plan, shares authorized	3,000,000								3,000,000
Stock issued during period, remaining shares available, dividend reinvestment plan	2,427,538								2,427,538
Dividends declared per common share	$ 0.24	$ 0.24	$ 0.24	$ 0.24	$ 0.25	$ 0.25	$ 0.25	$ 0.25
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	$ 8,829,000								$ 8,829,000
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	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