Commitments and Contingencies (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) category	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) category	Sep. 30, 2014 USD ($)
Loss Contingencies [Line Items]
Number of tenant and building improvement categories | category	2		2
Letters of credit outstanding	$ 400,000		$ 400,000
Line of Credit [Member] | Unsecured Debt [Member] | $500 Million Unsecured Line of Credit [Member]
Loss Contingencies [Line Items]
Amount of line of credit	500,000,000		500,000,000
Tenant Audits and Disputes [Member]
Loss Contingencies [Line Items]
Tenant audit dispute expense	0	$ 0	$ 100,000	$ 600,000
Non-Incremental Capital Expenditures [Member]
Loss Contingencies [Line Items]
Period for commitments for funding non-incremental capital expenditures			3 years
Potential obligations for tenant improvements	52,300,000		$ 52,300,000
Incremental Capital Expenditures [Member]
Loss Contingencies [Line Items]
Potential obligations for tenant improvements	$ 55,900,000		$ 55,900,000
Maximum [Member] | Non-Incremental Capital Expenditures [Member]
Loss Contingencies [Line Items]
Period for commitments for funding non-incremental capital expenditures			5 years