Acquisitions, Dispositions and Mortgage Repayments - Assets acquired and liabilities assumed (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Business Acquisition [Line Items]
Buildings	$ 280.2		$ 85.1
Land	21.6		13.5
Personal property	0.3
Prepaid ground leases			0.7
Intangibles
Total intangibles	14.5		6.2
Mortgage notes payable assumed, including fair value adjustment	(41.0)		(5.2)
Elimination of Construction Mortgage Note Receivable	(97.2)
Other assets acquired	1.2
Mortgage notes payable repayments			(9.9)
Accounts payable, accrued liabilities and other liabilities assumed	(1.9)		(0.9)
Total cash paid	177.7	[1]	89.5	[2]
Business Combination, Acquisition Related Costs	1.2		0.5
Prorated rent, net of expenses paid	1.2		0.4
Personal Property [Member]
Intangibles
Estimated useful life, buildings	1 year 10 months 24 days
Prepaid Ground Leases [Member]
Intangibles
Intangible asset useful life			54 years
At Market Lease Intangibles [Member]
Intangibles
Total intangibles	12.0		6.2
Above-market lease intangibles [Member]
Intangibles
Total intangibles	2.9
Below Market Lease Intangibles [Member]
Intangibles
Total intangibles	$ (0.4)
Minimum [Member] | Building [Member]
Intangibles
Estimated useful life, buildings	7 years		20 years
Minimum [Member] | Personal Property [Member]
Intangibles
Estimated useful life, buildings	1 year 10 months 24 days
Minimum [Member] | At Market Lease Intangibles [Member]
Intangibles
Intangible asset useful life	3 years 8 months 12 days		4 years 10 months 24 days
Minimum [Member] | Above-market lease intangibles [Member]
Intangibles
Intangible asset useful life	2 years 3 months 18 days
Minimum [Member] | Below Market Lease Intangibles [Member]
Intangibles
Intangible asset useful life	6 months
Maximum [Member] | Building [Member]
Intangibles
Estimated useful life, buildings	39 years		38 years
Maximum [Member] | Personal Property [Member]
Intangibles
Estimated useful life, buildings	15 years 9 months 18 days
Maximum [Member] | At Market Lease Intangibles [Member]
Intangibles
Intangible asset useful life	20 years		19 years 3 months 18 days
Maximum [Member] | Above-market lease intangibles [Member]
Intangibles
Intangible asset useful life	16 years 3 months 18 days
Maximum [Member] | Below Market Lease Intangibles [Member]
Intangibles
Intangible asset useful life	7 years 4 months 24 days

[1] Total cash paid includes receivables acquired and liabilities assumed in the acquisition but excludes acquisition and closing costs expensed totaling $1.2 million as well as rental prorations, net of expense disbursements, totaling approximately $1.2 million.

[2] Total cash paid includes receivables acquired and liabilities assumed in the acquisition but excludes acquisition and closing costs expensed totaling $0.5 million as well as rental prorations and expense disbursements totaling approximately $0.4 million.