Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Shareholders' Equity, Common Stock [Roll Forward]
Balance at beginning of period			54,268,915	40,382,530
Common stock issued under DRIP			505,718	6,947
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)	0
Balance at end of period	55,173,392	54,366,382	55,173,392	54,366,382
Dividend reinvestment plan, shares authorized	3,000,000		3,000,000
Dividends declared per common share	$ 0.29	$ 0.29	$ 0.58	$ 0.57
Additional Paid in Capital [Abstract]
Additional Paid in Capital, Common Stock beginning of period			$ 759,214	$ 634,683
Additional Paid In Capital from DRIP issuances of common stock			5,365	66
Adjustments related to tax withholding for share-based compensation			545	0
Amortization of restricted stock, net of additional grants			1,096	812
Additional Paid in Capital, Capitalized expenses			(59)	(119)
Additional Paid in Capital, Common Stock, end of period	767,172	759,167	767,172	759,167
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated other comprehensive income, available-for-sale securities, gross unrealized gains	50,839		50,839		104,869
Accumulated other comprehensive income, available-for-sale securities, gross unrealized losses	(71,381)		(71,381)		(12,623)
Accumulated other comprehensive income from available-for-sale securities, net unrealized gain	(20,542)		(20,542)		92,246
Accumulated Other Comprehensive Income (Loss), Unrealized Gains from Cash Flow Hedges, Effect Net of Tax	3,966		3,966		0
Accumulated other comprehensive income from hedging instruments, unrealized losses	(18,524)		(18,524)		(39,735)
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(14,558)		(14,558)		(39,735)
Accumulated other comprehensive income	(35,100)		(35,100)		52,511
Stock issued during period, remaining shares available, dividend reinvestment plan	2,476,437		2,476,437
At the market progam [Member]
Shareholders' Equity, Common Stock [Roll Forward]
Common stock issued under ATM program			180,986	402,494
Common stock issued via secondary offering			180,986	402,494
Additional Paid in Capital [Abstract]
Additional Paid in Capital from ATM issuances of common stock			1,954	3,674
Additional Paid in Capital from secondary offering of common stock			1,954	3,674
Secondary offering [Member]
Shareholders' Equity, Common Stock [Roll Forward]
Common stock issued under ATM program			0	13,332,748
Common stock issued via secondary offering			0	13,332,748
Additional Paid in Capital [Abstract]
Additional Paid in Capital from ATM issuances of common stock			0	119,859
Additional Paid in Capital from secondary offering of common stock			0	119,859
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends Per Share, Declared	$ 0.53125
Preferred Stock, Dividend Rate, Percentage	8.50%
Preferred Stock, Shares Issued	2,300,000		2,300,000		2,300,000
Preferred Stock, Par Value Per Share	$ 0.01		$ 0.01		$ 0.01
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
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Including Additional Paid in Capital, Net of Discount	$ 54,478		$ 54,478
Preferred Stock, Dividends Per Share, Declared	$ 0.46068
Preferred Stock, Dividend Rate, Percentage	7.625%
Preferred Stock, Shares Issued	2,250,000		2,250,000		0
Preferred Stock, Par Value Per Share	$ 0.01		$ 0.01		$ 0
Preferred Stock, Liquidation Preference Per Share	$ 25.00		$ 25.00
Preferred Shares Issued, Underwriters' Option [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued	250,000		250,000
Stock Appreciation Rights (SARs) [Member]
Class of Stock [Line Items]
Share-based compensation arrangement other than option, vested and outstanding, price per share	$ 7.06		$ 7.06