Schedule III - Real Estate and Accumulated Depreciation and Amortization - Notes to Schedule III (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at the beginning of year	$ 6,647,597	$ 5,913,547	$ 5,236,251
Acquisitions, completed construction and tenant improvements	744,577	833,764	717,899
Disposition of land, buildings, and leasehold interests	(73,089)	(91,818)	(36,633)
Provision for loss on impairment of real estate	(4,953)	(7,896)	(3,970)
Balance at the close of year	7,314,132	6,647,597	5,913,547
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at the beginning of year	742,467	624,607	513,175
Disposition of land, buildings, and leasehold interests	(14,035)	(16,286)	(7,377)
Depreciation and amortization expense	152,689	134,146	118,809
Balance at the close of year	881,121	742,467	624,607
Work in progress - improvements	41,920	24,057	$ 61,354
Mortgages payable, including unamortized premium and net of unamortized debt costs	13,300	$ 13,878
Operating Lease
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at the close of year	7,267,243
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at the close of year	880,235
Real estate, federal income tax basis	7,201,219
Direct Financing Lease
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at the close of year	0
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance at the close of year	0
Real estate, federal income tax basis	2,703
Mortgage Agreement
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Mortgages payable, including unamortized premium and net of unamortized debt costs	$ 15,151