Acquisitions, Dispositions and Mortgage Repayments - Dispositions (Details) $ in Millions													3 Months Ended		12 Months Ended
	Dec. 22, 2016 USD ($) ft²	Dec. 20, 2016 USD ($) ft²	Oct. 28, 2016 USD ($) ft²	Oct. 14, 2016 USD ($) ft²	Dec. 14, 2015 USD ($) ft²	Nov. 05, 2015 USD ($) ft²	Sep. 30, 2015 USD ($) ft²	Sep. 25, 2015 USD ($) ft²	Sep. 16, 2015 USD ($) ft²	Jul. 17, 2015 USD ($) ft²	Jun. 30, 2015 USD ($) ft² property	May 21, 2015 USD ($) ft²	Sep. 30, 2015 USD ($) ft² property	Jun. 30, 2015 USD ($) ft² property	Dec. 31, 2016 USD ($) ft² property	Dec. 31, 2015 USD ($) ft² property
Real Estate Dispositions [Line Items]
Sales Price															$ 94.7
Closing Adjustments															(1.4)
Net Proceeds															93.3
Net Real Estate Investment															49.4
Other (including receivables)															2.9
Gain/ (Impairment)															$ 41.0
Square Footage | ft²															320,702
Number of properties sold | property													1	2	6	2
Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Sales Price																$ 158.0
Closing Adjustments																(4.9)
Net Proceeds																153.1
Net Real Estate Investment																82.9
Other (including receivables)																3.3
Gain/ (Impairment)																$ 66.9
Square Footage | ft²																534,074
Mortgage note repayments [Member]
Real Estate Dispositions [Line Items]
Mortgage Notes																$ 1.9
Kansas [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed				Oct. 14, 2016
Sales Price				$ 15.1
Closing Adjustments				0.0
Net Proceeds				15.1
Net Real Estate Investment				7.2
Other (including receivables)				0.3
Gain/ (Impairment)				$ 7.6
Square Footage | ft²				70,908
Arizona [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed		Dec. 20, 2016				Nov. 05, 2015		Sep. 25, 2015
Sales Price		$ 22.3				$ 5.3		$ 3.0
Closing Adjustments		0.0				(0.3)
Net Proceeds		22.3				5.0		3.0
Net Real Estate Investment		13.5				4.9		2.0
Other (including receivables)		1.4						0.2
Gain/ (Impairment)		$ 7.4				$ 0.1		$ 0.8
Square Footage | ft²		51,903				56,645		40,782
Florida [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed									Sep. 16, 2015
Sales Price									$ 16.3
Closing Adjustments									(0.5)
Net Proceeds									15.8
Net Real Estate Investment									10.5
Other (including receivables)									0.2
Gain/ (Impairment)									$ 5.1
Square Footage | ft²									119,903
Georgia [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed					Dec. 14, 2015
Sales Price					$ 14.0
Closing Adjustments					(0.2)
Net Proceeds					13.8
Net Real Estate Investment					4.4
Other (including receivables)					0.4
Gain/ (Impairment)					$ 9.0
Square Footage | ft²					58,030
Indiana [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed											Jun. 30, 2015
Sales Price											$ 97.0			$ 97.0
Closing Adjustments											(3.7)			(3.7)
Net Proceeds											93.3
Net Real Estate Investment											50.5			50.5
Other (including receivables)											1.9			$ 1.9
Gain/ (Impairment)											$ 40.9
Square Footage | ft²											175,999			175,999
Number of properties sold | property											2
Missouri [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed							Sep. 30, 2015
Sales Price							$ 3.0						$ 3.0
Closing Adjustments							(0.2)						(0.2)
Net Proceeds							2.8
Net Real Estate Investment							2.9						2.9
Other (including receivables)							0.2						$ 0.2
Gain/ (Impairment)							$ (0.3)
Square Footage | ft²							13,478						13,478
Pennsylvania [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed										Jul. 17, 2015
Sales Price										$ 18.4
Closing Adjustments										0.0
Net Proceeds										18.4
Net Real Estate Investment										7.4
Other (including receivables)										0.4
Gain/ (Impairment)										$ 10.6
Square Footage | ft²										63,914
Tennessee [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed	Dec. 22, 2016		Oct. 28, 2016
Sales Price	$ 2.8		$ 8.8
Closing Adjustments	(0.2)		(0.2)
Net Proceeds	2.6		8.6
Net Real Estate Investment	1.8		6.3
Other (including receivables)	0.0		0.2
Gain/ (Impairment)	$ 0.8		$ 2.1
Square Footage | ft²	8,749		45,274
Altoona, Pennsylvania [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed		Dec. 20, 2016
Sales Price		$ 21.5
Closing Adjustments		(0.4)
Net Proceeds		21.1
Net Real Estate Investment		12.4
Other (including receivables)		0.6
Gain/ (Impairment)		$ 8.1
Square Footage | ft²		64,032
Harrisburg, Pennsylvania [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed		Dec. 20, 2016
Sales Price		$ 24.2
Closing Adjustments		(0.6)
Net Proceeds		23.6
Net Real Estate Investment		8.2
Other (including receivables)		0.4
Gain/ (Impairment)		$ 15.0
Square Footage | ft²		79,836
Virginia [Member] | Real estate dispositions [Member]
Real Estate Dispositions [Line Items]
Date Disposed												May 21, 2015
Sales Price												$ 1.0
Closing Adjustments												0.0
Net Proceeds												1.0
Net Real Estate Investment												0.3
Gain/ (Impairment)												$ 0.7
Square Footage | ft²												5,323