Indebtedness (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Oct. 31, 2014	Dec. 31, 2013	May 31, 2014	Apr. 30, 2014	Jul. 31, 2012	Dec. 31, 2011
Indebtedness
Face Amount	$ 1,750,000		$ 1,100,000
Unamortized Discount	6,373		6,663
Total secured	627,076		699,427
Interest rate (as a percent)		5.875%
Initial Cost of Collateral	6,238,611		5,263,625
Net Book Value of Collateral	5,254,761		4,422,865
Unamortized net premiums	2,937		4,485
Net book value of the assets encumbered by capital leases	18,237		18,627
Required principal payments on outstanding debt
2015	71,541
2016	410,660
2017	65,941
2018	96,197
2019	671,757
Thereafter	1,488,043
Unsecured Debt
Indebtedness
Face Amount	350,000			350,000
Interest rate (as a percent)	1.60%
Senior unsecured notes 4.30%
Indebtedness
Face Amount	250,000		250,000
Unamortized Discount	551		1,085
Interest rate (as a percent)	4.30%
Senior unsecured notes 3.25%
Indebtedness
Face Amount	400,000				400,000
Unamortized Discount	256
Interest rate (as a percent)	3.25%				3.25%
Senior unsecured notes 6.75%
Indebtedness
Face Amount	200,000		200,000			350,000	300,000
Unamortized Discount	1,133		1,348
Interest rate (as a percent)	6.75%
Unsecured senior notes due 2021
Indebtedness
Face Amount	300,000		300,000
Unamortized Discount	3,696		4,230
Interest rate (as a percent)	6.75%
Senior unsecured notes 4.75%
Indebtedness
Face Amount	250,000				250,000
Unamortized Discount	737
Interest rate (as a percent)	4.75%				4.75%
Senior unsecured notes 5.625% due in 2042
Indebtedness
Face Amount	350,000		350,000
Interest rate (as a percent)	5.625%
Secured and Other Debt
Indebtedness
Total secured	624,139		694,942
Number of properties as collateral	47
Initial Cost of Collateral	1,419,482
Net Book Value of Collateral	834,302		964,047
Mortgage 5.83% maturing in June 2014
Indebtedness
Total secured			36,145
Interest rate (as a percent)	5.83%
Initial Cost of Collateral	79,000
Net Book Value of Collateral			77,799
Mortgage 6.015% maturing in March 2015
Indebtedness
Total secured	29,362		30,177
Interest rate (as a percent)	6.015%
Number of properties as collateral	1
Initial Cost of Collateral	99,000
Net Book Value of Collateral	94,245		96,354
Mortgage 6.25% maturing in May 2015
Indebtedness
Total secured			12,093
Interest rate (as a percent)	6.25%
Initial Cost of Collateral	22,350
Net Book Value of Collateral			22,033
Mortgage 5.65% maturing in June 2015
Indebtedness
Total secured	4,914		5,020
Interest rate (as a percent)	5.65%
Number of properties as collateral	1
Initial Cost of Collateral	22,143
Net Book Value of Collateral	20,511		20,965
Mortgage 6.37% maturing in July 2015
Indebtedness
Total secured			11,465
Interest rate (as a percent)	6.37%
Initial Cost of Collateral	14,849
Net Book Value of Collateral			14,197
Mortgage 5.66% maturing in July 2015
Indebtedness
Total secured	12,479		12,773
Interest rate (as a percent)	5.66%
Number of properties as collateral	3
Initial Cost of Collateral	26,606
Net Book Value of Collateral	24,948		25,457
Mortgage 5.880% maturing in July 2015
Indebtedness
Total secured	2,728		2,805
Interest rate (as a percent)	5.88%
Number of properties as collateral	1
Initial Cost of Collateral	15,397
Net Book Value of Collateral	13,991		14,384
Mortgage 5.81% maturing in October 2015
Indebtedness
Total secured	6,353		6,579
Interest rate (as a percent)	5.81%
Number of properties as collateral	1
Initial Cost of Collateral	9,650
Net Book Value of Collateral	9,927		9,474
Mortgage 5.810% maturing in October 2015, Two
Indebtedness
Total secured	4,403		4,502
Interest rate (as a percent)	5.81%
Number of properties as collateral	1
Initial Cost of Collateral	8,600
Net Book Value of Collateral	8,211		8,168
Mortgage 5.64% maturing in January 2016
Indebtedness
Total secured	52,000		52,000
Interest rate (as a percent)	5.64%
Number of properties as collateral	1
Initial Cost of Collateral	70,495
Net Book Value of Collateral	64,175		64,904
Mortgage 5.97% maturing in April 2016
Indebtedness
Total secured	6,243		6,363
Interest rate (as a percent)	5.97%
Number of properties as collateral	1
Initial Cost of Collateral	10,272
Net Book Value of Collateral	9,552		9,811
Mortgage 5.92% maturing in November 2016
Indebtedness
Total secured	85,085		87,928
Interest rate (as a percent)	5.92%
Number of properties as collateral	2
Initial Cost of Collateral	157,500
Net Book Value of Collateral	150,036		151,928
Mortgage 6.25% maturing in November 2016
Indebtedness
Total secured	12,184		12,366
Interest rate (as a percent)	6.25%
Number of properties as collateral	1
Initial Cost of Collateral	22,102
Net Book Value of Collateral	21,255		21,672
Mortgage 5.86% maturing in March 2017
Indebtedness
Total secured	5,625		5,720
Interest rate (as a percent)	5.86%
Number of properties as collateral	1
Initial Cost of Collateral	11,280
Net Book Value of Collateral	11,004		11,292
Mortgages 6.54% maturing in May 2017
Indebtedness
Total secured	44,687		45,753
Interest rate (as a percent)	6.54%
Number of properties as collateral	8
Initial Cost of Collateral	62,500
Net Book Value of Collateral	53,878		54,702
Mortgages 6.150% maturing in August 2017
Indebtedness
Total secured	11,059		11,245
Interest rate (as a percent)	6.15%
Number of properties as collateral	1
Initial Cost of Collateral	16,400
Net Book Value of Collateral	14,846		15,180
Mortgages 6.73% maturing in April 2018
Indebtedness
Total secured	9,195		9,425
Interest rate (as a percent)	6.73%
Number of properties as collateral	1
Initial Cost of Collateral	15,100
Net Book Value of Collateral	11,103		11,299
Mortgage 6.71% maturing in Sept 2019
Indebtedness
Total secured	288,519		292,611
Interest rate (as a percent)	6.71%
Number of properties as collateral	17
Initial Cost of Collateral	617,161
Net Book Value of Collateral	245,593		252,404
Mortgage 7.31% and 7.85% maturing in Jan 2022
Indebtedness
Number of mortgages	2
Mortgage 7.31% maturing in Jan 2022
Indebtedness
Total secured	2,723		3,007
Interest rate (as a percent)	7.31%
Number of properties as collateral	1
Initial Cost of Collateral	18,827
Net Book Value of Collateral	16,136		16,478
Mortgage 7.85% maturing in Jan 2022
Indebtedness
Total secured	1,345		1,482
Interest rate (as a percent)	7.85%
Mortgage 6.28% maturing in July 2022
Indebtedness
Total secured	15,318
Interest rate (as a percent)	6.28%
Number of properties as collateral	1
Initial Cost of Collateral	32,658
Net Book Value of Collateral	25,848
Mortgage 6.25% maturing in February 2033
Indebtedness
Total secured	3,348		3,444
Interest rate (as a percent)	6.25%
Initial Cost of Collateral	5,200
Net Book Value of Collateral	4,483		4,588
Mortgage 5.95% maturing in August 2037
Indebtedness
Total secured	9,205		9,353
Interest rate (as a percent)	5.95%
Number of properties as collateral	1
Initial Cost of Collateral	11,425
Net Book Value of Collateral	8,865		9,080
Mortgage 4.38% maturing in September 2043
Indebtedness
Total secured	4,594		4,672
Interest rate (as a percent)	4.375%
Number of properties as collateral	1
Initial Cost of Collateral	8,059
Net Book Value of Collateral	7,459		7,632
Mortgage bonds
Indebtedness
Total secured			14,700
Interest rate (as a percent)	5.875%
Initial Cost of Collateral	34,307
Net Book Value of Collateral			25,619
Capital leases
Indebtedness
Total secured	12,770		13,314
Interest rate (as a percent)	7.70%
Number of properties as collateral	2
Initial Cost of Collateral	28,601
Net Book Value of Collateral	$ 18,237		$ 18,627