Gain on Sales of Real Estate (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) property
Real Estate Properties [Line Items]
Number of properties sold \| property	128	59	77
Proceeds from sales of real estate	$ 142,286	$ 166,976	$ 99,096
Gain on sales of properties	$ 24,600	$ 40,900	22,000
Properties
Real Estate Properties [Line Items]
Proceeds from sales of real estate			90,500
Former Corporate Headquarters
Real Estate Properties [Line Items]
Proceeds from sales of real estate			$ 8,600

The difference between the carrying value and the sale price of real estate or properties that were intended to be sold or held for capital appreciation or rental income. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109226691&loc=d3e2941-110230

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2443-110228

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration