Schedule III - Real Estate and Accumulated Depreciation and Amortization Notes to Schedule III (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at the beginning of year	$ 3,531,845	$ 2,774,947	$ 2,584,947
Acquisitions, completed construction and tenant improvements	701,054	772,073	248,438
Disposition of land, buildings, and leasehold interests	(77,219)	(14,744)	(58,438)
Provision for loss on impairment of real estate	(10,312)	(431)	0
Balance at the close of year	4,145,368	3,531,845	2,774,947
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at the beginning of year	270,621	222,921	183,949
Disposition of land, buildings, and leasehold interests	(6,980)	(3,010)	(2,071)
Depreciation and amortization expense	70,137	50,710	41,043
Balance at the close of year	333,778	270,621	222,921
Work in process	86,579	60,322	26,699
Mortgages payable, including unamortized premium of $187 and $0, respectively	10,602	23,171
Operating Lease [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Real estate, federal income tax basis	4,020,556
Direct Financing Lease [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Real estate, federal income tax basis	4,178
Mortgage Agreement-2 [Member]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Mortgages payable, including unamortized premium of $187 and $0, respectively	$ 6,952