Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Shareholders' Equity, Common Stock [Roll Forward]
Balance at beginning of period			54,268,915
Common stock issued under DRIP			509,831
Common stock issued or redeemed under Stock and Incentive Plans			270,158
Common stock forfeited for tax withholding on share-based compensation			(52,385)
Stock Repurchased During Period, Shares			751,456
Balance at end of period	54,426,049		54,426,049
Dividend reinvestment plan, shares authorized	3,000,000		3,000,000
Dividends declared per common share	$ 0.27	$ 0.29	$ 0.85	$ 0.86
Additional Paid in Capital [Abstract]
Additional Paid in Capital, Common Stock beginning of period			$ 759,214
Additional Paid In Capital from DRIP issuances of common stock			5,401
Adjustments related to tax withholding for share-based compensation			545
Stock Repurchased During Period, Value			(5,956)
Amortization of restricted stock, net of additional grants			1,736
Additional Paid in Capital, Capitalized expenses			(89)
Additional Paid in Capital, Common Stock, end of period	761,862		761,862
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated other comprehensive income, available-for-sale securities, gross unrealized gains	49,669		49,669		104,869
Accumulated other comprehensive income, available-for-sale securities, gross unrealized losses	(72,057)		(72,057)		(12,623)
Accumulated other comprehensive income from available-for-sale securities, net unrealized gain	(22,388)		(22,388)		92,246
Accumulated Other Comprehensive Income (Loss), Unrealized Gains from Cash Flow Hedges, Effect Net of Tax	3,779		3,779		0
Accumulated other comprehensive income from hedging instruments, unrealized losses	(15,754)		(15,754)		(39,735)
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(11,975)		(11,975)		(39,735)
Accumulated other comprehensive income	(34,363)		(34,363)		52,511
Stock Repurchase Program, Authorized Amount			50,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount			43,115
Stock issued during period, remaining shares available, dividend reinvestment plan	2,472,324		2,472,324
At the market progam [Member]
Shareholders' Equity, Common Stock [Roll Forward]
Common stock issued under ATM program			180,986
Additional Paid in Capital [Abstract]
Additional Paid in Capital from ATM issuances of common stock			1,954
Stock Compensation Plan [Member]
Additional Paid in Capital [Abstract]
Additional Paid in Capital from ATM issuances of common stock			$ 147
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
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividends Per Share, Declared	$ 0.4765625
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
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
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