Newly Issued Accounting Pronouncements (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) center capital_lease administrative_office lease	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retrospective reclassification from cash flows provided by (used in) operating activities	$ 269,758	$ 252,411	$ 245,435
Reclassification to cash flows (used in) provided by financing activities	(588,695)	129,798	(126,248)
Reclassification to cash flows (used in) provided by investing activities	298,992	(366,172)	(197,132)
Acquisition related costs		1,400
Accumulated other comprehensive loss	$ (6,170)	(9,161)
Number of lessor leases | lease	5,400
Number of properties subject to ground leases | center	13
Operating leases, number of administrative offices | administrative_office	3
Operating leases, rental expense	$ 2,900	3,000	3,200
Capital leases, number | capital_lease	1
Capital leased assets, number of units | center	2
Obligations under capital leases	$ 21,000	21,000
Leasing deferred costs	10,800	10,300
Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retrospective reclassification from cash flows provided by (used in) operating activities		6,000	1,800
Reclassification to cash flows (used in) provided by financing activities		(6,000)	(1,800)
Accounting Standards Update 2016-15 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retrospective reclassification from cash flows provided by (used in) operating activities		(500)	(800)
Reclassification to cash flows (used in) provided by investing activities		500	800
Accounting Standards Update 2016-18 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) in restricted cash		20,000	(76,600)
Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Defined benefit plan, net periodic benefit cost (credit), excluding the service cost component	$ 400	$ 700	$ 200
Scenario, Forecast [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in retained earnings				$ 300
Contract assets				300
Scenario, Forecast [Member] | Accounting Standards Update 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in retained earnings				1,500
Accumulated other comprehensive loss				(1,500)
Scenario, Forecast [Member] | Accounting Standards Update 2017-05 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in retained earnings				4,000
Other assets				$ 4,000