Shareholders' Equity (Details) (USD $)	3 Months Ended		9 Months Ended				9 Months Ended				3 Months Ended	9 Months Ended					0 Months Ended	1 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Aug. 01, 2012	Dec. 31, 2011	Sep. 30, 2012 At the market progam/ATM [Member]	Sep. 30, 2012 Secondary offering [Member]	Sep. 30, 2012 Common Stock [Member]	Sep. 30, 2011 Common Stock [Member]	Sep. 30, 2012 Common Stock [Member] At the market progam/ATM [Member]	Sep. 30, 2012 Common Stock [Member] At the market progam/ATM [Member]	Sep. 30, 2011 Common Stock [Member] At the market progam/ATM [Member]	Sep. 30, 2012 Common Stock [Member] Secondary offering [Member]	Sep. 30, 2011 Common Stock [Member] Secondary offering [Member]	Sep. 30, 2012 Common Stock [Member] Option Exercised by Underwriter [Member]	Aug. 01, 2012 Nonrecognized Subsequent Event [Member]	Aug. 06, 2012 Nonrecognized Subsequent Event [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued	2,300,000		2,300,000			0											2,300,000
Preferred Stock, Dividend Rate, Percentage																		8.50%
Preferred Stock, Par or Stated Value Per Share	$ 0.01		$ 0.01		$ 0.01	$ 0.00
Preferred Stock, Liquidation Preference Per Share																	$ 25.00
Proceeds from Issuance of Preferred Stock and Preference Stock																	$ 55,689,000
Shareholders' Equity, Common Stock [Roll Forward]
Balance at beginning of period			40,382,530						40,382,530	30,342,897
Common Stock Issued											0	402,494	409,237	13,332,748	9,200,000	832,487
Common stock issued under DRIP									9,279	2,111
Common stock issued or redeemed under Stock and Incentive Plans									241,663	426,031
Balance at end of period	54,368,714		54,368,714						54,368,714	40,380,276
Equity Placement Program, Shares Authorized												8,000,000
Proceeds from Issuance of Common Stock			123,832,000	95,261,000								3,721		119,992
Sales Commissions and Fees												57
Equity Placement Program, Shares Sold, Average Price per Share												$ 9.39
Common Stock, Dividends, Per Share, Declared	$ 0.29	$ 0.27	$ 0.86	$ 0.81
Stock Issued During Period, New Issues, Price per Share														$ 9.12
Additional Paid in Capital [Abstract]
Balance as of January 1, 2012	759,647,000		759,647,000			634,683,000
DRIP issuances			90,000
Stock Issued During Period, Value, New Issues							3,674,000	119,859,000
Incentive plans			192,000
Amortization of restricted stock			1,268,000
Capitalized expenses			(119,000)
Balance as of September 30, 2012	759,647,000		759,647,000
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Available-for-sale securities, gross unrealized gains	110,332,000		110,332,000			36,091,000
Available-for-sale Securities, Gross Unrealized Losses	(10,747,000)		(10,747,000)			(13,902,000)
Available-for-sale Securities, Gross Unrealized Gain (Loss)	99,585,000		99,585,000			22,189,000
Hedging Instruments, Loss Recognized in Other Comprehensive Income (Loss), Effective Portion	(43,695,000)		(43,695,000)			(25,444,000)
Hedging Instruments, Gain (loss) in accumulated other comprehensive (loss) income	(43,695,000)		(43,695,000)			(25,444,000)
Accumulated other comprehensive (loss) income	$ 55,890,000		$ 55,890,000			$ (3,255,000)