Summary of Significant Accounting Policies - Concentration of credit risk (Details) (USD $) In Millions, unless otherwise specified					0 Months Ended	6 Months Ended
	Jun. 30, 2012 Tenants financial_institutions	Jun. 30, 2012 Healthcare industry Texas Tenants	Jun. 30, 2012 Distribution industry Illinois Tenants	Jun. 30, 2012 Automotive industry Ohio Tenants	Jun. 29, 2012 Credit concentration risk Cash on deposit	Jun. 30, 2012 Credit concentration risk Gross annualized rental revenues by industry Healthcare industry Texas	Jun. 30, 2012 Credit concentration risk Gross annualized rental revenues by industry Distribution industry Illinois	Jun. 30, 2012 Credit concentration risk Gross annualized rental revenues by industry Automotive industry Ohio
Concentration Risk [Line Items]
Cash on deposit, number of financial institutions which had deposits in excess of current federally insured limits	3
Cash on deposit in excess of federally insured levels					$ 39.0
Number of tenants	3	1	1	1
Concentration risk, percentage						79.00%	9.00%	12.00%