Intangible Assets and Liabilities - Components of intangible assets and liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Schedule of finite lived intangible assets and liabilities
Intangible assets and liabilities, gross	$ 148.3	$ 144.2
Intangible assets and liabilities, accumulated amortization	$ 60.4	47.7
Assets and liabilities, weighted avg. life (years)	16 years 6 months
Below-market lease intangibles [Member] | Other liabilities [Member]
Schedule of finite lived intangible assets and liabilities
Liabilities gross balance	$ (9.3)	(9.5)
Liabilities accumulated amortization	$ (4.1)	(3.5)
Liabilities, weighted avg. life (years)	38 years
Debt issuance costs [Member] | Notes and bonds payable [Member]
Schedule of finite lived intangible assets and liabilities
Assets gross balance	$ 9.3	9.3
Assets accumulated amortization	$ 2.8	1.8
Assets, weighted avg. life (years)	3 years 8 months 12 days
Goodwill [Member] | Other assets [Member]
Schedule of finite lived intangible assets and liabilities
Assets gross balance	$ 3.5	3.5
Assets accumulated amortization	0.0	0.0
Debt issuance costs [Member] | Other assets [Member]
Schedule of finite lived intangible assets and liabilities
Assets gross balance	5.4	5.4
Assets accumulated amortization	$ 3.2	1.9
Assets, weighted avg. life (years)	1 year 7 months 6 days
Above-market lease intangibles [Member] | Other assets [Member]
Schedule of finite lived intangible assets and liabilities
Assets gross balance	$ 22.3	22.9
Assets accumulated amortization	$ 5.5	5.0
Assets, weighted avg. life (years)	59 years 7 months 6 days
Customer relationship intangibles [Member] | Other assets [Member]
Schedule of finite lived intangible assets and liabilities
Assets gross balance	$ 2.7	2.6
Assets accumulated amortization	$ 1.0	0.9
Assets, weighted avg. life (years)	24 years 7 months 6 days
At market lease Intangibles [Member] | Real estate properties [Member]
Schedule of finite lived intangible assets and liabilities
Assets gross balance	$ 114.4	110.0
Assets accumulated amortization	$ 52.0	$ 41.6
Assets, weighted avg. life (years)	4 years 9 months 18 days