Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
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-Subparagraph (a)(3)
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The number of real estate properties owned as of the balance sheet date.

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- Definition

The portion of the deferred gain that was recognized in the current period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 40
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The amount of the gain recorded in connection with the transaction involving the sale of property to another party and the lease of the property back to the seller to be recognized in future periods.

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The portion of the gain that will be recorded as income or a reduction in rent expense in future periods less amounts recognized in the current period.

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Amount of cash inflow before closing and debt issuance costs received by a seller-lessee in a sale-leaseback recognized in investing activities.

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-Publisher FASB
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Lease Obligations - Sale and Lease Back of Buildings (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015 USD ($) building	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) building
Sale lease back transaction
Proceeds from sale of buildings				$ 82,082
Sale and leaseback of office buildings
Sale lease back transaction
Number of buildings \| building	2			2
Proceeds from sale of buildings				$ 82,000
Term of lease				12 years
Remaining deferred gain will be amortized over the remaining life of the lease	$ 43,000			$ 43,000
Sale and leaseback of office buildings \| Corporate general and administrative expense
Sale lease back transaction
Gain on sale of property	58,000			$ 58,000
Gain on sale of property recognized	$ 7,000	$ 4,000	$ 4,000