Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended	1 Months Ended	3 Months Ended												12 Months Ended
	Aug. 01, 2012	Aug. 06, 2012	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, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Class of Stock [Line Items]
Preferred Stock, Shares Issued	2,300,000		2,300,000				0								2,300,000	0
Preferred Stock, Dividend Rate, Percentage		8.50%
Preferred Stock, Par or Stated Value Per Share	$ 0.01		$ 0.01				$ 0.00								$ 0.01	$ 0.00
Preferred Stock, Liquidation Preference Per Share	$ 25.00
Proceeds from Issuance of Preferred Stock and Preference Stock	$ 55,689
Shareholders' Equity, Common Stock [Roll Forward]
Balance at beginning of period						40,382,530									40,382,530
Balance at end of period			54,268,915				40,382,530								54,268,915	40,382,530
Proceeds from Issuance of Common Stock															123,868	95,282	116,850
Stock Repurchase Program, Authorized Amount															50,000
Dividend reinvestment plan, shares authorized			3,000,000												3,000,000
Dividends declared per common share			$ 0.29	$ 0.29	$ 0.29	$ 0.28	$ 0.28	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.25	$ 0.23	$ 0.23	$ 1.15	$ 1.09	$ 0.98
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Available-for-sale securities, gross unrealized gains			104,870				36,091								104,870	36,091
Available-for-sale Securities, Gross Unrealized Losses			(12,623)				(13,902)								(12,623)	(13,902)
Available-for-sale Securities, Gross Unrealized Gain (Loss)			92,247				22,189								92,247	22,189
Hedging Instruments, Loss Recognized in Other Comprehensive Income (Loss), Effective Portion			(39,736)				(25,444)								(39,736)	(25,444)
Hedging Instruments, Gain (loss) in accumulated other comprehensive (loss) income			(39,736)				(25,444)								(39,736)	(25,444)
Accumulated other comprehensive (loss) income			52,511				(3,255)								52,511	(3,255)
Stock Repurchased and Retired During Period, Shares															(104,000)	0
Stock repurchase price															$ 8.86
Preferred Stock, Dividends Per Share, Declared			$ 0.53125	$ 0.43681
Common Stock [Member]
Shareholders' Equity, Common Stock [Roll Forward]
Balance at beginning of period						40,382,530				30,342,897					40,382,530	30,342,897
Common stock issued under DRIP															13,480	4,365
Common stock issued or redeemed under Stock and Incentive Plans															241,663	426,031
Balance at end of period			54,268,915				40,382,530								54,268,915	40,382,530
Common Stock [Member] | At the market progam [Member]
Shareholders' Equity, Common Stock [Roll Forward]
Common Stock Issued															402,494	409,237
Equity Placement Program, Shares Authorized															8,000,000
Proceeds from Issuance of Common Stock															3,721
Sales Commissions and Fees															57
Equity Placement Program, Shares Sold, Average Price per Share															$ 9.39
Common Stock [Member] | Secondary offering [Member]
Shareholders' Equity, Common Stock [Roll Forward]
Common Stock Issued															13,332,748	9,200,000
Proceeds from Issuance of Common Stock															$ 119,992
Stock Issued During Period, New Issues, Price per Share															$ 9.12
Common Stock [Member] | Option Exercised by Underwriter [Member]
Shareholders' Equity, Common Stock [Roll Forward]
Common Stock Issued															832,487