Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended						3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 At the market progam [Member]	Mar. 31, 2013 At the market progam [Member]	Mar. 31, 2014 Series A Preferred Stock [Member]	Dec. 31, 2013 Series A Preferred Stock [Member]	Mar. 31, 2014 Series B Preferred Stock [Member]	Dec. 31, 2013 Series B Preferred Stock [Member]	Mar. 31, 2014 Common Stock [Member] At the market progam [Member]	Mar. 31, 2014 Restricted Stock [Member]	Mar. 31, 2014 Four to five years [Member] Restricted Stock [Member]
Class of Stock [Line Items]
Nonvested restricted stock, fair value remaining to be amortized										$ 6,092	$ 541
Preferred Stock, Dividend Rate, Percentage					8.50%		7.625%
Preferred Stock, Liquidation Preference Per Share	$ 25.00
Preferred Stock, Shares Issued					2,300,000	2,300,000	2,250,000	2,250,000
Shareholders' Equity, Common Stock [Roll Forward]
Balance at beginning of period	54,310,484	54,268,915
Common stock issued under DRIP	3,938	298,304
Common stock issued under ATM program			0	180,986
Common stock issued or redeemed under Stock and Incentive Plans	442,035	139,600
Common stock forfeited for tax withholding on share-based compensation	(59,150)	(52,385)
Balance at end of period	54,697,307	54,835,420
ATM program, authorized shares remaining									7,416,520
Dividend reinvestment plan, shares authorized	3,000,000
Dividends declared per common share	$ 0.25
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	2,462,960