Acquisitions, Dispositions and Mortgage Repayments - Dispositions (Details) $ in Thousands												3 Months Ended	12 Months Ended
	Sep. 07, 2017 USD ($) ft²	Jun. 29, 2017 USD ($) ft²	Jun. 16, 2017 USD ($) ft²	Mar. 31, 2017 USD ($) ft²	Mar. 30, 2017 USD ($) ft²	Mar. 07, 2017 USD ($) ft²	Mar. 06, 2017 USD ($) ft²	Dec. 22, 2016 USD ($) ft²	Dec. 20, 2016 USD ($) ft²	Oct. 28, 2016 USD ($) ft²	Oct. 14, 2016 USD ($) ft²	Mar. 31, 2017 USD ($) ft²	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Oct. 31, 2017 USD ($)
Real Estate Dispositions [Line Items]
Sales Price													$ 122,700	$ 94,700		$ 45,500
Closing Adjustments													(3,300)	(1,400)
Net Proceeds													119,400	93,300
Net Real Estate Investment													78,200	49,400
Other (including receivables)													6,800	2,900
Gain/ (Impairment)													$ 34,400	$ 41,000
Square Footage | ft²													416,481	320,702
Impairment of real estate													$ 5,385		$ 3,639
Indiana [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed							Mar. 06, 2017
Sales Price							$ 6,400
Closing Adjustments							0
Net Proceeds							6,400
Net Real Estate Investment							1,100
Other (including receivables)							0
Gain/ (Impairment)							$ 5,300
Square Footage | ft²							29,500
Georgia [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed						Mar. 07, 2017		Dec. 22, 2016
Sales Price						$ 600		$ 2,800
Closing Adjustments						0		(200)
Net Proceeds						600		2,600
Net Real Estate Investment						600		1,800
Other (including receivables)						0		0
Gain/ (Impairment)						$ 0		$ 800
Square Footage | ft²						12,000		8,749
Nevada [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed					Mar. 30, 2017
Sales Price					$ 5,500
Closing Adjustments					(700)
Net Proceeds					4,800
Net Real Estate Investment					2,200
Other (including receivables)					300
Gain/ (Impairment)					$ 2,300
Square Footage | ft²					18,147
Texas [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed				Mar. 31, 2017
Sales Price				$ 69,500								$ 69,500
Closing Adjustments				(1,600)								(1,600)
Net Proceeds				67,900
Net Real Estate Investment				46,900								46,900
Other (including receivables)				5,200								$ 5,200
Gain/ (Impairment)				$ 15,800
Square Footage | ft²				169,722								169,722
Illinois [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed			Jun. 16, 2017
Sales Price			$ 500
Closing Adjustments			(100)
Net Proceeds			400
Net Real Estate Investment			400
Other (including receivables)			0
Gain/ (Impairment)			$ 0
Square Footage | ft²			5,100
Impairment of real estate												$ 300
San Antonio, Texas [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed		Jun. 29, 2017
Sales Price		$ 14,500
Closing Adjustments		(200)
Net Proceeds		14,300
Net Real Estate Investment		5,100
Other (including receivables)		900
Gain/ (Impairment)		$ 8,300
Square Footage | ft²		39,786
Oregon [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed		Jun. 29, 2017
Sales Price		$ 23,200
Closing Adjustments		(600)
Net Proceeds		22,600
Net Real Estate Investment		14,500
Other (including receivables)		300
Gain/ (Impairment)		$ 7,800
Square Footage | ft²		62,246
Missouri [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed	Sep. 07, 2017
Sales Price	$ 2,500
Closing Adjustments	(100)
Net Proceeds	2,400
Net Real Estate Investment	7,400
Other (including receivables)	100
Gain/ (Impairment)	$ (5,100)
Square Footage | ft²	79,980
Kansas [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed											Oct. 14, 2016
Sales Price											$ 15,100
Closing Adjustments											0
Net Proceeds											15,100
Net Real Estate Investment											7,200
Other (including receivables)											300
Gain/ (Impairment)											$ 7,600
Square Footage | ft²											70,908
Tennessee [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed										Oct. 28, 2016
Sales Price										$ 8,800
Closing Adjustments										(200)
Net Proceeds										8,600
Net Real Estate Investment										6,300
Other (including receivables)										200
Gain/ (Impairment)										$ 2,100
Square Footage | ft²										45,274
Altoona, Pennsylvania [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed									Dec. 20, 2016
Sales Price									$ 21,500
Closing Adjustments									(400)
Net Proceeds									21,100
Net Real Estate Investment									12,400
Other (including receivables)									600
Gain/ (Impairment)									$ 8,100
Square Footage | ft²									64,032
Harrisburg, Pennsylvania [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed									Dec. 20, 2016
Sales Price									$ 24,200
Closing Adjustments									(600)
Net Proceeds									23,600
Net Real Estate Investment									8,200
Other (including receivables)									400
Gain/ (Impairment)									$ 15,000
Square Footage | ft²									79,836
Arizona [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed									Dec. 20, 2016
Sales Price									$ 22,300
Closing Adjustments									0
Net Proceeds									22,300
Net Real Estate Investment									13,500
Other (including receivables)									1,400
Gain/ (Impairment)									$ 7,400
Square Footage | ft²									51,903