Intangible Assets and Liabilities - Components of intangible assets and liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of finite lived intangible assets and liabilities
Intangible assets and liabilities, gross	$ 144.2	$ 120.7
Intangible assets and liabilities, accumulated amortization	$ 47.7	46.5
Assets and liabilities, weighted avg. life (years)	15 years 2 months
Below-market lease intangibles [Member] | Other liabilities [Member]
Schedule of finite lived intangible assets and liabilities
Liabilities gross balance	$ (9.5)	(8.8)
Liabilities accumulated amortization	$ (3.5)	(3.2)
Liabilities, weighted avg. life (years)	36 years 6 months
Deferred financing costs [Member] | Other liabilities [Member]
Schedule of finite lived intangible assets and liabilities
Liabilities gross balance	$ 9.3	9.4
Liabilities accumulated amortization	$ 1.8	4.0
Liabilities, weighted avg. life (years)	4 years 6 months
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
Deferred financing costs [Member] | Other assets [Member]
Schedule of finite lived intangible assets and liabilities
Assets gross balance	5.4	5.4
Assets accumulated amortization	$ 1.9	0.5
Assets, weighted avg. life (years)	2 years 7 months 6 days
Above-market lease intangibles [Member] | Other assets [Member]
Schedule of finite lived intangible assets and liabilities
Assets gross balance	$ 22.9	24.5
Assets accumulated amortization	$ 5.0	5.4
Assets, weighted avg. life (years)	57 years 9 months 18 days
Customer relationship intangibles [Member] | Other assets [Member]
Schedule of finite lived intangible assets and liabilities
Assets gross balance	$ 2.6	2.6
Assets accumulated amortization	$ 0.9	0.8
Assets, weighted avg. life (years)	25 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	$ 110.0	84.1
Assets accumulated amortization	$ 41.6	$ 39.0
Assets, weighted avg. life (years)	5 years 7 months 6 days