Indebtedness (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended																																					12 Months Ended																									1 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2012 Unsecured Debt	Dec. 31, 2011 Unsecured Debt	Jan. 31, 2012 Senior unsecured notes 8.625%	Dec. 31, 2011 Senior unsecured notes 8.625%	Dec. 31, 2012 Senior unsecured notes 4.30%	Dec. 31, 2011 Senior unsecured notes 4.30%	Jan. 31, 2011 Senior unsecured notes 4.30%	Dec. 31, 2012 Senior unsecured notes 6.75%	Jul. 31, 2012 Senior unsecured notes 6.75%	Dec. 31, 2011 Senior unsecured notes 6.75%	Apr. 30, 2010 Senior unsecured notes 6.75%	Dec. 31, 2012 Unsecured senior notes due 2021	Dec. 31, 2011 Unsecured senior notes due 2021	Dec. 31, 2012 Senior unsecured notes 5.625% due in 2042	Jul. 31, 2012 Senior unsecured notes 5.625% due in 2042	Dec. 31, 2012 Secured and Other Debt Property	Dec. 31, 2011 Secured and Other Debt	Dec. 31, 2012 Mortgage 6.03% maturing in March 2012	Dec. 31, 2011 Mortgage 6.03% maturing in March 2012	Dec. 31, 2012 Mortgage 6.73% maturing in June 2012	Dec. 31, 2011 Mortgage 6.73% maturing in June 2012	Dec. 31, 2012 Mortgages 6.97% maturing in July 2012	Dec. 31, 2011 Mortgages 6.97% maturing in July 2012	Dec. 31, 2012 Mortgage 6.07% maturing in September 2012	Dec. 31, 2011 Mortgage 6.07% maturing in September 2012	Dec. 31, 2012 Mortgage 6.50% maturing in Jan 2013	Dec. 31, 2011 Mortgage 6.50% maturing in Jan 2013	Dec. 31, 2012 Mortgages 6.11% maturing in Dec 2013 Property	Dec. 31, 2011 Mortgages 6.11% maturing in Dec 2013	Dec. 31, 2012 Mortgage 6.91% maturing in Dec 2013 Property	Dec. 31, 2011 Mortgage 6.91% maturing in Dec 2013	Dec. 31, 2012 Mortgage 5.83% maturing in June 2014 Property	Dec. 31, 2011 Mortgage 5.83% maturing in June 2014	Dec. 31, 2012 Mortgage 6.015% maturing in March 2015 Property	Dec. 31, 2011 Mortgage 6.015% maturing in March 2015	Dec. 31, 2012 Mortgage 5.65% maturing in June 2015 item Property	Dec. 31, 2011 Mortgage 5.65% maturing in June 2015	Dec. 31, 2012 Mortgage 6.365% maturing in July 2015 Property	Dec. 31, 2011 Mortgage 6.365% maturing in July 2015	Dec. 31, 2012 Mortgage 5.66% maturing in July 2015 Property	Dec. 31, 2011 Mortgage 5.66% maturing in July 2015	Dec. 31, 2012 Mortgage 5.88% maturing in July 2015 Property	Dec. 31, 2011 Mortgage 5.88% maturing in July 2015	Dec. 31, 2012 Mortgage 5.81% maturing in October 2015, Two Property	Dec. 31, 2012 Mortgage 5.81% maturing in October 2015 Property	Dec. 31, 2011 Mortgage 5.81% maturing in October 2015	Dec. 31, 2012 Mortgage 5.64% maturing in January 2016 Property	Dec. 31, 2011 Mortgage 5.64% maturing in January 2016	Dec. 31, 2012 Mortgage 5.97% maturing in April 2016 Property	Dec. 31, 2011 Mortgage 5.97% maturing in April 2016	Dec. 31, 2012 Mortgage 5.924% maturing in November 2016 Property	Dec. 31, 2011 Mortgage 5.924% maturing in November 2016	Dec. 31, 2012 Mortgage 6.25% maturing in November 2016 Property	Dec. 31, 2011 Mortgage 6.25% maturing in November 2016	Dec. 31, 2012 Mortgage 5.86% maturing in March 2017 Property	Dec. 31, 2011 Mortgage 5.86% maturing in March 2017	Dec. 31, 2012 Mortgages 6.54% maturing in May 2017 Property	Dec. 31, 2011 Mortgages 6.54% maturing in May 2017	Dec. 31, 2012 Mortgages 6.15% maturing in August 2017 Property	Dec. 31, 2012 Mortgages 6.73% maturing in April 2018 Property	Aug. 31, 2009 Mortgages 6.71% and 6.39% maturing in Sept 2019 item Property	Aug. 31, 2012 Mortgages 6.71% and 6.39% maturing in Sept 2019 item	Dec. 31, 2012 Mortgage 6.71% maturing in Sept 2019 Property	Dec. 31, 2011 Mortgage 6.71% maturing in Sept 2019	Dec. 31, 2012 Mortgage 6.39% maturing in Sept 2019	Dec. 31, 2011 Mortgage 6.39% maturing in Sept 2019	Dec. 31, 2012 Mortgage 7.31% maturing in Jan 2022 Property	Dec. 31, 2011 Mortgage 7.31% maturing in Jan 2022	Jul. 31, 2008 Mortgage 7.31% maturing in Jan 2022 item	Dec. 31, 2012 Mortgage 7.85% maturing in Jan 2022	Dec. 31, 2011 Mortgage 7.85% maturing in Jan 2022	Dec. 31, 2012 Mortgage 6.25% maturing in February 2033 Property	Dec. 31, 2011 Mortgage 6.25% maturing in February 2033	Dec. 31, 2012 Mortgage 5.95% maturing in September 2038 Property	Dec. 31, 2011 Mortgage 5.95% maturing in September 2038	Dec. 31, 2012 Mortgage 4.375% maturing in September 2043 Property	Dec. 31, 2011 Mortgage 4.375% maturing in September 2043	Dec. 31, 2012 Mortgage bonds Property	Dec. 31, 2011 Mortgage bonds	Dec. 31, 2012 Capital leases Property	Dec. 31, 2011 Capital leases	Dec. 31, 2012 Mortgage 6.07% maturing in Sept 2012 and 5.65% maturing in June 2015 item
Indebtedness
Face Amount			$ 1,100,000	$ 975,000		$ 225,000	$ 250,000	$ 250,000		$ 200,000	$ 350,000	$ 200,000	$ 200,000	$ 300,000	$ 300,000	$ 350,000
Unamortized Discount			7,947	9,230			1,620	2,154		1,563		1,777		4,764	5,299
Total secured	724,477	861,615																719,047	861,424		12,400		2,356		30,580		3,177		4,224	10,565	10,920	13,759	14,009	36,906	37,619	30,944		5,121	5,215	11,612	11,747	13,051	13,310	2,878	2,946	6,792	4,596	4,684	52,000		6,476	6,581	90,607	93,133	12,537	12,695	5,810		46,753	47,688	11,419	9,641			296,437	300,669		200,078	3,270	3,515		1,608	1,725	3,534	3,619	9,492	9,623	4,747		14,700	14,700	13,792	14,211
Interest rate (as a percent)					8.625%	8.625%	4.30%	4.30%	4.30%	6.75%		6.75%		6.75%	6.75%		5.625%			6.03%	6.03%	6.73%	6.73%	6.97%	6.97%	6.07%	6.07%	6.50%	6.50%	6.11%	6.11%	6.91%	6.91%	5.83%	5.83%	6.015%	6.015%	5.65%	5.65%	6.365%	6.365%	5.66%	5.66%	5.88%	5.88%	5.81%	5.81%	5.81%	5.64%	5.64%	5.97%	5.97%	5.924%	5.924%	6.25%	6.25%	5.86%	5.86%	6.54%	6.54%	6.15%	6.73%			6.71%	6.71%	6.39%	6.39%	7.31%	7.31%		7.85%	7.85%	6.25%	6.25%	5.95%	5.95%	4.375%	4.375%	5.875%	5.875%	7.70%	7.70%
Number of Properties as Collateral																		58												4		2		2		1		1		1		3		1		1	1		1		1		2		1		1		8		1	1	28		17				1					1		2		1		1		2
Initial Cost of Collateral	5,183,307	4,721,591																1,517,149		17,158		4,450		70,114		22,143		7,560		17,034		36,359		79,000		99,000				14,849		26,606		15,397		9,650	8,600		70,495		10,272		157,500		22,102		11,280		62,500		16,400	15,100			617,161				18,827					5,200		11,425		8,059		34,307		28,601
Net Book Value of Collateral	4,432,404	4,091,330																1,008,246	1,142,035		17,076		3,696		61,542				7,406	14,883	14,814	33,057	33,702	78,594	79,108	98,346		21,435	21,838	14,432	14,697	25,868	26,377	14,774	15,173	9,665	8,391	8,532	66,123		10,051	10,194	154,691	157,640	21,968	21,984	11,562		56,341	57,873	15,507	11,526			260,274	505,214			16,839	17,243				4,695	4,807	9,295	19,589	7,768		26,559	27,503	15,602	16,027
Unamortized net premiums	5,430	191
Number of mortgage loans repaid	20																																					1
Number of Properties released from mortgage as collateral																																																																11
Number of mortgages																																																																							2													2
Net book value of the assets encumbered by capital leases	13,792	14,211
Number of mortgages closed																																																															2
Required principal payments on outstanding debt
2013	36,109
2014	48,022
2015	272,356
2016	410,136
2017	65,382
Thereafter	$ 1,177,042