Acquisitions, Dispositions and Mortgage Repayments - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Building	$ 64.0	$ 272.1
Land	31.4	11.7
Land Improvements	2.7	1.6
Intangibles	14.2	36.7
Mortgage notes payable assumed, including fair value adjustments	(8.0)	(46.4)
Other assets acquired	0.2	0.4
Accounts payable, accrued liabilities and other liabilities assumed	(1.4)	(1.7)
Equity investment in joint ventures		8.7
Total cash paid	103.1	283.1
At market lease Intangibles [Member]
Business Acquisition [Line Items]
Intangibles	14.3	37.2
Below-market lease intangibles [Member]
Business Acquisition [Line Items]
Intangible liabilities	$ (0.1)	(0.9)
Intangible asset useful life	3 years 9 months 18 days
Below-market ground lease intangibles [Member]
Business Acquisition [Line Items]
Intangibles		$ 0.4
Minimum [Member] | At market lease Intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life	1 year 3 months 18 days	2 years 1 month 6 days
Minimum [Member] | Below-market lease intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life		8 years 6 months
Minimum [Member] | Below-market ground lease intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life		36 years 9 months 18 days
Minimum [Member] | Building [Member]
Business Acquisition [Line Items]
Asset estimated useful life	20 years	15 years
Minimum [Member] | Land Improvements [Member]
Business Acquisition [Line Items]
Asset estimated useful life	5 years	5 years
Maximum [Member] | At market lease Intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life	4 years 2 months 12 days	12 years 7 months 6 days
Maximum [Member] | Below-market lease intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life		15 years
Maximum [Member] | Below-market ground lease intangibles [Member]
Business Acquisition [Line Items]
Intangible asset useful life		99 years
Maximum [Member] | Building [Member]
Business Acquisition [Line Items]
Asset estimated useful life	30 years	37 years
Maximum [Member] | Land Improvements [Member]
Business Acquisition [Line Items]
Asset estimated useful life	13 years 6 months	12 years