Acquisitions, Dispositions, Discontinued Operations and Assets Held for Sale Acquisitions, Dispositions, Discontinued Operations and Assets Held for Sale (Assets and Liabilities Held for Sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Long Lived Assets Held-for-sale [Line Items]
Land	$ 998,519	$ 989,247
Buildings and improvements	5,978,843	5,911,432
Real estate assets, at cost, total	6,977,362	6,900,679
Accumulated depreciation	(1,841,107)	(1,780,694)
Net operating real estate assets	5,136,255	5,119,985
Properties under development, including land	489,730	486,918
Properties held for sale, including land	238,417	239,063
Discontinued Operations, Held-for-sale or Disposed of by Sale [Member]
Long Lived Assets Held-for-sale [Line Items]
Land	59,438	59,438
Buildings and improvements	377,100	373,419
Real estate assets, at cost, total	436,538	432,857
Accumulated depreciation	(202,323)	(197,996)
Net operating real estate assets	234,215	234,861
Properties under development, including land	4,202	4,202
Properties held for sale, including land	238,417	239,063
Other assets related to properties held for sale, including land	1,139	1,191
Total assets held for sale	239,556	240,254
Liabilities related to assets held for sale	$ 1,371	$ 1,654