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	89,691				89,691
Other comprehensive (loss) income	87,023			87,023
Issuance of common stock:
Dividend reinvestment & stock purchase plans (in shares)		2,609,151
Dividend reinvestment & stock purchase plans	36,424	26	36,398
Employee stock purchase and incentive plans (in shares)		361,262
Employee stock purchase and incentive plans	(1,902)	3	(1,905)
Non-cash equity award compensation	7,369		7,369
Common dividends declared	(61,337)					(61,337)
Ending balance at Sep. 30, 2012	1,049,852	815	1,739,841	73,872	590,974	(1,355,650)
Ending balance (in shares) at Sep. 30, 2012		81,526,321
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	148,112				148,112
Other comprehensive (loss) income	(23,564)			(23,564)
Issuance of common stock:
Dividend reinvestment & stock purchase plans (in shares)		374,371
Dividend reinvestment & stock purchase plans	7,077	4	7,073
Employee stock purchase and incentive plans (in shares)		297,715
Employee stock purchase and incentive plans	(5,387)	3	(5,390)
Non-cash equity award compensation	11,443		11,443
Common dividends declared	(70,675)					(70,675)
Ending balance at Sep. 30, 2013	$ 1,207,170	$ 824	$ 1,757,680	$ 114,768	$ 781,164	$ (1,447,266)
Ending balance (in shares) at Sep. 30, 2013		82,388,502