Acquisitions, Dispositions and Mortgage Repayments - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Building	$ 272.1	$ 216.8
Land	11.7	9.7
Land Improvements	1.6
Intangibles	36.7	13.7
Mortgage notes payable assumed, including fair value adjustments	(46.4)	(14.0)
Other assets acquired	0.4	0.5
Equity investment in joint ventures	8.7
Accounts payable, accrued liabilities and other liabilities assumed	(1.7)	(1.8)
Total cash paid	283.1	224.9
At market lease Intangibles [Member]
Business Acquisition [Line Items]
Intangibles	37.2	12.8
Above-market lease intangibles [Member]
Business Acquisition [Line Items]
Intangibles		0.9
Below-market ground lease intangibles [Member]
Business Acquisition [Line Items]
Intangibles	$ 0.4	$ 2.0
Minimum [Member] \| At market lease Intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life	2 years 1 month 6 days	2 years 8 months 12 days
Minimum [Member] \| Above-market lease intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life		8 months 12 days
Minimum [Member] \| Below-market lease intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life	8 years 6 months	1 year 4 months 24 days
Minimum [Member] \| Below-market ground lease intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life	36 years 9 months 18 days	36 years 9 months 18 days
Minimum [Member] \| Building [Member]
Business Acquisition [Line Items]
Asset estimated useful life	15 years	20 years
Minimum [Member] \| Land Improvements [Member]
Business Acquisition [Line Items]
Asset estimated useful life	5 years
Maximum [Member] \| At market lease Intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life	12 years 7 months 6 days	10 years 3 months 18 days
Maximum [Member] \| Above-market lease intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life		3 years 9 months 18 days
Maximum [Member] \| Below-market lease intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life	15 years	9 years 4 months 24 days
Maximum [Member] \| Below-market ground lease intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life	99 years	99 years
Maximum [Member] \| Building [Member]
Business Acquisition [Line Items]
Asset estimated useful life	37 years	35 years
Maximum [Member] \| Land Improvements [Member]
Business Acquisition [Line Items]
Asset estimated useful life	39 years
Above-market ground lease intangibles [Member]
Business Acquisition [Line Items]
Intangible liabilities		$ (1.6)
Intangible asset useful life		99 years
Below-market lease intangibles [Member]
Business Acquisition [Line Items]
Intangible liabilities	$ (0.9)	$ (0.4)
2017 Acquisitions [Member] \| Minimum [Member] \| Land Improvements [Member]
Business Acquisition [Line Items]
Asset estimated useful life	5 years
2017 Acquisitions [Member] \| Maximum [Member] \| Land Improvements [Member]
Business Acquisition [Line Items]
Asset estimated useful life	12 years

Business acquisition purchase price allocation mortgage notes payable.

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- Definition

Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Equity Investment in Joint Ventures

+ References

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- Definition

Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangible Liabilities

+ References

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- Definition

Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Land Improvements

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Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets

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- Definition

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- Definition

Amount of facility held for productive use including, but not limited to, office, production, storage and distribution facilities, acquired at the acquisition date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
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Amount of liabilities due within one year or within the normal operating cycle, if longer, assumed at the acquisition date.

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- Definition

The amount of identifiable intangible assets recognized as of the acquisition date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 55
-Paragraph 37
-URI http://asc.fasb.org/extlink&oid=84160947&loc=d3e2207-128464

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Amount of real estate acquired, at the acquisition date.

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- Definition

Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed.

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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