Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share	$ 25.00												$ 25.00
Shareholders' Equity, Common Stock [Roll Forward]
Balance at beginning of period				54,268,915				40,382,530					54,268,915	40,382,530
Common stock issued under DRIP													515,257	13,480
Common stock issued or redeemed under Stock and Incentive Plans													270,158	241,663
Common stock forfeited for tax withholding on share-based compensation													(52,385)
Stock Repurchased During Period, Shares													(872,447)	(104,000)
Balance at end of period	54,310,484				54,268,915				40,382,530				54,310,484	54,268,915	40,382,530
Dividend reinvestment plan, shares authorized	3,000,000												3,000,000
Dividends declared per common share	$ 0.27	$ 0.27	$ 0.29	$ 0.29	$ 0.29	$ 0.29	$ 0.29	$ 0.28	$ 0.28	$ 0.27	$ 0.27	$ 0.27	$ 1.1200	$ 1.1500	$ 1.0900
Stock Repurchase Program, Authorized Amount													$ 50,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount													$ 42,145
Stock issued during period, remaining shares available, dividend reinvestment plan	2,466,898												2,466,898
At the market progam [Member]
Shareholders' Equity, Common Stock [Roll Forward]
Common stock issued under ATM program													180,986	402,494
Secondary offering [Member]
Shareholders' Equity, Common Stock [Roll Forward]
Common stock issued under ATM program														13,332,748
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				0								2,250,000	0
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
Stock Appreciation Rights (SARs) [Member]
Class of Stock [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	27,500												27,500
Share-based compensation arrangement other than option, vested and outstanding, price per share	$ 7.06												$ 7.06
Ordinary income [Member]
Shareholders' Equity, Common Stock [Roll Forward]
Dividends declared per common share													$ 1.0650	$ 1.0038	$ 0.9822
Capital Gain [Member]
Shareholders' Equity, Common Stock [Roll Forward]
Dividends declared per common share													$ 0.0550	$ 0.1462	$ 0.1078