New Issued Accounting Pronouncements (Details) $ in Thousands	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) center capital_lease administrative_office lease	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of lessor leases | lease		5,100
Number of properties with ground lease agreements | center		12
Number of administrative office leases | administrative_office		3
Operating leases, rent expense, net		$ 1,300	$ 1,500
Number of capital leases | capital_lease		1
Capital leases assets, number of units | center		2
Obligations under capital leases		$ 21,000		$ 21,000
Capitalized internal costs		$ 4,700		9,500
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change on equity or net assets					$ 300
Accounting Standards Update 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change on equity or net assets					1,500
Accounting Standards Update 2017-05 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change on equity or net assets					$ 3,600
Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit cost, excluding the service cost component	$ 100		$ 200	$ 400