Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Jun. 30, 2013	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Class of Stock [Line Items]
Preferred Stock, Dividends Per Share, Declared		$ 0.53125
Preferred Stock, Dividend Rate, Percentage	7.625%	8.50%
Shareholders' Equity, Common Stock [Roll Forward]
Balance at beginning of period	54,835,420	54,268,915	40,382,530
Common stock issued under DRIP		298,304	3,932
Common stock issued or redeemed under Stock and Incentive Plans		139,600	216,663
Common stock forfeited for tax withholding on share-based compensation		(52,385)	0
Balance at end of period		54,835,420	54,338,367
Proceeds from Issuance of Common Stock		$ 5,152	$ 123,733
Dividend reinvestment plan, shares authorized		3,000,000
Dividends declared per common share		$ 0.29	$ 0.28
Additional Paid in Capital [Abstract]
Additional Paid in Capital, Common Stock beginning of period	764,272	759,214	634,683
Additional Paid In Capital from DRIP issuances of common stock		3,224	37
Adjustments related to tax withholding for share-based compensation		545	0
Amortization of restricted stock, net of additional grants		455	369
Additional Paid in Capital, Capitalized expenses		(30)	(119)
Additional Paid in Capital, Common Stock, end of period		764,272	758,503
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated other comprehensive income, available-for-sale securities, gross unrealized gains		112,641		104,870
Accumulated other comprehensive income, available-for-sale securities, gross unrealized losses		(15,888)		(12,623)
Accumulated other comprehensive income from available-for-sale securities, net unrealized gain		96,753		92,247
Accumulated other comprehensive income from hedging instruments, unrealized losses		(35,196)		(39,736)
Accumulated other comprehensive income		61,557		52,511
Stock issued during period, remaining shares available, dividend reinvestment plan		2,688,216
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
Common Stock [Member] | At the market progam [Member]
Shareholders' Equity, Common Stock [Roll Forward]
ATM program, authorized shares remaining		7,416,520
Proceeds from Issuance of Common Stock		1,956
Stock broker commission		$ 30