New Issued Accounting Pronouncements (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) center capital_lease administrative_office lease	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of lessor leases | lease	5,200
Number of properties with ground lease agreements | center	13
Number of administrative office leases | administrative_office	3
Operating leases, rent expense, net	$ 700	$ 800
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	$ 2,500		10,800
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	$ 400