Acquisitions, Dispositions and Mortgage Repayments - Dispositions (Details) $ in Millions															3 Months Ended	12 Months Ended
	Dec. 30, 2019 USD ($) ft²	Dec. 18, 2019 USD ($) ft²	Nov. 27, 2019 USD ($) ft²	Nov. 25, 2019 USD ($) ft²	Oct. 25, 2019 USD ($) ft²	Aug. 28, 2019 USD ($) ft²	Aug. 01, 2019 USD ($) ft²	Apr. 09, 2019 USD ($) ft²	Dec. 27, 2018 USD ($) ft²	Dec. 21, 2018 USD ($) ft²	Dec. 20, 2018 USD ($) ft²	Aug. 30, 2018 USD ($) ft²	Jun. 27, 2018 USD ($) ft² property	Apr. 26, 2018 USD ($) ft² building property	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) ft²	Dec. 31, 2018 USD ($) ft²
Real Estate Dispositions [Line Items]
SALES PRICE																$ 54.9	$ 98.7
CLOSING ADJUSTMENTS																(2.5)	(1.9)
NET PROCEEDS																52.4	96.8
Net Real Estate Investment																25.8	54.5
OTHER including receivables																1.5	2.5
GAIN/ (IMPAIRMENT)																$ 25.1	$ 39.8
SQUARE FOOTAGE unaudited | ft²																537,050	806,206
Number of properties exercising purchase option | property														7
Number of single-tenant net leased buildings | building														5
Number of multi-tenant buildings | building														2
Arizona [Member] | Real Estate Dispositions [Member]
Real Estate Dispositions [Line Items]
DATE DISPOSED								Apr. 09, 2019	Dec. 27, 2018
SALES PRICE								$ 13.0	$ 3.0
CLOSING ADJUSTMENTS								(0.9)	(0.2)
NET PROCEEDS								12.1	2.8
Net Real Estate Investment								6.9	1.9
OTHER including receivables								0.4	0.2
GAIN/ (IMPAIRMENT)								$ 4.8	$ 0.7
SQUARE FOOTAGE unaudited | ft²								67,345	37,310
Virginia [Member] | Real Estate Dispositions [Member]
Real Estate Dispositions [Line Items]
DATE DISPOSED							Aug. 01, 2019							Apr. 26, 2018
SALES PRICE							$ 1.3							$ 46.2
CLOSING ADJUSTMENTS							(0.1)							0.0
NET PROCEEDS							1.2							46.2
Net Real Estate Investment							1.2							23.9
OTHER including receivables							0.0							0.0
GAIN/ (IMPAIRMENT)							$ 0.0							$ 22.3
SQUARE FOOTAGE unaudited | ft²							10,000							460,881
Impairment charge															$ 0.4
San Antonio, Texas [Member] | Real Estate Dispositions [Member]
Real Estate Dispositions [Line Items]
DATE DISPOSED						Aug. 28, 2019
SALES PRICE						$ 0.9
CLOSING ADJUSTMENTS						(0.1)
NET PROCEEDS						0.8
Net Real Estate Investment						0.6
OTHER including receivables						0.0
GAIN/ (IMPAIRMENT)						$ 0.2
SQUARE FOOTAGE unaudited | ft²						10,138
Erie, ‎Pennsylvania [Member] | Real Estate Dispositions [Member]
Real Estate Dispositions [Line Items]
DATE DISPOSED					Oct. 25, 2019
SALES PRICE					$ 14.0
CLOSING ADJUSTMENTS					0.0
NET PROCEEDS					14.0
Net Real Estate Investment					1.3
OTHER including receivables					0.0
GAIN/ (IMPAIRMENT)					$ 12.7
SQUARE FOOTAGE unaudited | ft²					90,123
Louisiana [Member] | Real Estate Dispositions [Member]
Real Estate Dispositions [Line Items]
DATE DISPOSED				Nov. 25, 2019
SALES PRICE				$ 3.7
CLOSING ADJUSTMENTS				(0.2)
NET PROCEEDS				3.5
Net Real Estate Investment				1.2
OTHER including receivables				0.2
GAIN/ (IMPAIRMENT)				$ 2.1
SQUARE FOOTAGE unaudited | ft²				136,155
Tennessee [Member] | Real Estate Dispositions [Member]
Real Estate Dispositions [Line Items]
DATE DISPOSED			Nov. 27, 2019							Dec. 21, 2018
SALES PRICE			$ 9.5							$ 13.3
CLOSING ADJUSTMENTS			(0.3)							(0.2)
NET PROCEEDS			9.2							13.1
Net Real Estate Investment			5.0							8.6
OTHER including receivables			1.3							0.4
GAIN/ (IMPAIRMENT)			$ 2.9							$ 4.1
SQUARE FOOTAGE unaudited | ft²			75,000							81,382
Pittsburgh, ‎Pennsylvania [Member] | Real Estate Dispositions [Member]
Real Estate Dispositions [Line Items]
DATE DISPOSED		Dec. 18, 2019
SALES PRICE		$ 3.8
CLOSING ADJUSTMENTS		(0.3)
NET PROCEEDS		3.5
Net Real Estate Investment		3.5
OTHER including receivables		0.0
GAIN/ (IMPAIRMENT)		$ 0.0
SQUARE FOOTAGE unaudited | ft²		78,731
Impairment charge															$ 5.2
Dallas, Texas [Member] | Real Estate Dispositions [Member]
Real Estate Dispositions [Line Items]
DATE DISPOSED	Dec. 30, 2019
SALES PRICE	$ 8.7
CLOSING ADJUSTMENTS	(0.6)
NET PROCEEDS	8.1
Net Real Estate Investment	6.1
OTHER including receivables	(0.4)
GAIN/ (IMPAIRMENT)	$ 2.4
SQUARE FOOTAGE unaudited | ft²	69,558
Michigan [Member] | Real Estate Dispositions [Member]
Real Estate Dispositions [Line Items]
DATE DISPOSED													Jun. 27, 2018
SALES PRICE													$ 9.5
CLOSING ADJUSTMENTS													(0.7)
NET PROCEEDS													8.8
Net Real Estate Investment													3.4
OTHER including receivables													0.0
GAIN/ (IMPAIRMENT)													$ 5.4
SQUARE FOOTAGE unaudited | ft²													121,672
Number of properties exercising purchase option | property													5
Forfeited earnest money													$ 0.5
Missouri [Member] | Real Estate Dispositions [Member]
Real Estate Dispositions [Line Items]
DATE DISPOSED												Aug. 30, 2018
SALES PRICE												$ 9.8
CLOSING ADJUSTMENTS												(0.5)
NET PROCEEDS												9.3
Net Real Estate Investment												7.5
OTHER including receivables												0.5
GAIN/ (IMPAIRMENT)												$ 1.3
SQUARE FOOTAGE unaudited | ft²												70,893
Colorado [Member] | Real Estate Dispositions [Member]
Real Estate Dispositions [Line Items]
DATE DISPOSED											Dec. 20, 2018
SALES PRICE											$ 16.9
CLOSING ADJUSTMENTS											(0.3)
NET PROCEEDS											16.6
Net Real Estate Investment											9.2
OTHER including receivables											1.4
GAIN/ (IMPAIRMENT)											$ 6.0
SQUARE FOOTAGE unaudited | ft²											34,068