Consolidated Statements Of Changes In Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Cumulative Earnings	Cumulative Distributions to Stockholders
Beginning balance at Dec. 31, 2011	$ 892,584	$ 786	$ 1,697,979	$ (13,151)	$ 501,283	$ (1,294,313)
Beginning balance (in shares) at Dec. 31, 2011		78,555,908
Net Income	49,990				49,990
Other comprehensive (loss) income	41,363			41,363
Issuance of common stock:
Dividend reinvestment & stock purchase plans (in shares)		384,005
Dividend reinvestment & stock purchase plans	4,433	4	4,429
Employee stock purchase and incentive plans (in shares)		322,915
Employee stock purchase and incentive plans	(1,911)	3	(1,914)
Non-cash equity award compensation	5,217		5,217
Common dividends declared	(40,498)					(40,498)
Ending balance at Jun. 30, 2012	951,178	793	1,705,711	28,212	551,273	(1,334,811)
Ending balance (in shares) at Jun. 30, 2012		79,262,828
Beginning balance at Dec. 31, 2012	1,140,164	817	1,744,554	138,332	633,052	(1,376,591)
Beginning balance (in shares) at Dec. 31, 2012		81,716,416
Net Income	126,183				126,183
Other comprehensive (loss) income	(20,049)			(20,049)
Issuance of common stock:
Dividend reinvestment & stock purchase plans (in shares)		321,120
Dividend reinvestment & stock purchase plans	6,046	3	6,043
Employee stock purchase and incentive plans (in shares)		294,200
Employee stock purchase and incentive plans	(5,451)	3	(5,454)
Non-cash equity award compensation	9,515		9,515
Common dividends declared	(47,095)					(47,095)
Ending balance at Jun. 30, 2013	$ 1,209,313	$ 823	$ 1,754,658	$ 118,283	$ 759,235	$ (1,423,686)
Ending balance (in shares) at Jun. 30, 2013		82,331,736