Intangible Assets and Liabilities - Components of intangible assets and liabilities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Schedule of finite lived intangible assets and liabilities
Intangible assets and liabilities, gross	$ 166.3	$ 149.5
Intangible assets and liabilities, accumulated amortization	94.3	80.3
Assets and liabilities, weighted avg. life (years)	17 years
Below-market lease intangibles [Member] | Other liabilities [Member]
Schedule of finite lived intangible assets and liabilities
Liabilities gross balance	(9.2)	(8.9)
Liabilities accumulated amortization	(4.8)	(4.1)
Liabilities, weighted avg. life (years)	14 years 3 months 18 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
Deferred financing costs [Member] | Other assets [Member]
Schedule of finite lived intangible assets and liabilities
Assets gross balance	17.6	16.4
Assets accumulated amortization	7.7	4.7
Assets, weighted avg. life (years)	4 years 3 months 18 days
Above-market lease intangibles [Member] | Other assets [Member]
Schedule of finite lived intangible assets and liabilities
Assets gross balance	21.1	17.3
Assets accumulated amortization	3.8	2.9
Assets, weighted avg. life (years)	52 years 8 months 12 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	0.7	0.6
Assets, weighted avg. life (years)	28 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	130.7	118.6
Assets accumulated amortization	$ 86.9	$ 76.2
Assets, weighted avg. life (years)	7 years 2 months 12 days