Summary of Significant Accounting Policies (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Deferred Costs
Amortization of deferred financing fees	$ 0.3	$ 0.3
Tenant Accounts Receivable, net
Allowance for doubtful accounts	0		0
Accrued rental revenue	7.2		6.4
Allowance on accrued rental revenue	0		0
Lease security deposits available in existing letters of credit	5.4		4.8
Lease security deposits available in cash	2.0		2.0
Revenue Recognition
Estimated amount of real estate taxes, which are the responsibility of tenants	$ 2.3	$ 1.5