Shareholders' Equity (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period			40,382,530
Balance at end of period	54,366,382		54,366,382
Proceeds from Issuance of Common Stock			$ 123,808,000	$ 95,242,000
Common Stock, Dividends, Per Share, Declared	$ 0.29	$ 0.27	$ 0.57	$ 0.54
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share-based Compensation Expense	468	258	920	353
Additional Paid in Capital [Abstract]
Balance as of January 1, 2012	759,167,000		759,167,000		634,683,000
DRIP issuances			66,000
Incentive plans			192,000
Amortization of restricted stock			812,000
Capitalized expenses			(119,000)
Balance as of June 30, 2012	759,167,000		759,167,000
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Available-for-sale securities, gross unrealized gains	65,279,000		65,279,000		36,091,000
Available-for-sale Securities, Gross Unrealized Losses	(7,325,000)		(7,325,000)		(13,902,000)
Available-for-sale Securities, Gross Unrealized Gain (Loss)	57,954,000		57,954,000		22,189,000
Hedging Instruments, Loss Recognized in Other Comprehensive Income (Loss), Effective Portion	(36,449,000)		(36,449,000)		(25,444,000)
Hedging Instruments, Gain (loss) in accumulated other comprehensive (loss) income	(36,449,000)		(36,449,000)		(25,444,000)
Accumulated other comprehensive (loss) income	21,505,000		21,505,000		(3,255,000)
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Restricted stock at the beginning of period	457,034	320,500	365,506	25,000
Restricted stock granted	20,000	20,000	220,821	323,000
Restricted stock vested	(22,917)	(10,000)	(132,210)	(17,500)
Restricted stock outstanding at end of period	454,117	330,500	454,117	330,500
Outstanding restricted stock remaining amortization	3,894		3,894
Stock Appreciation Rights (SARs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
SARs Outstanding	52,500		52,500
SAR's weighted average price	$ 6.85		$ 6.85
Weighted average remaining contractual term			13 months
Outstanding SARs set to expire if not exercised	25,000		25,000
Fair value of Company's oustanding SARs	164		164		77
At the market progam/ATM [Member]
Additional Paid in Capital [Abstract]
Stock Issued During Period, Value, New Issues			3,674,000
Secondary offering [Member]
Additional Paid in Capital [Abstract]
Stock Issued During Period, Value, New Issues			119,859,000
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period			40,382,530	30,342,897
Common stock issued under DRIP			6,947	0
Common stock issued or redeemed under Stock and Incentive Plans			241,663	391,025
Balance at end of period	54,366,382	40,343,159	54,366,382	40,343,159
Common stock shares authorized under incentive plan	2,500,000		2,500,000
Common Stock, Shares Authorized	1,805,276		1,805,276
Common Stock [Member] | At the market progam/ATM [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock Issued	0		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]
Increase (Decrease) in Stockholders' Equity [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 Excercised by Underwriter [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock Issued			832,487