Real Estate Mortgages Receivable (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	9 Months Ended
	Jan. 31, 2013 property	Sep. 30, 2013	Dec. 31, 2012
Real Estate Mortgages Receivable
Number of properties sold	3
Number of buildings sold	18
Mortgage financing related to suburban property sold	$ 7,688
Interest rate on real estate mortgage receivable (as a percent)	6.00%
Amount received from prepayment of real estate mortgage receivable		1,000
Total real estate mortgage receivable included in other assets, carrying value		$ 8,107	$ 1,419