Segment Information Segment Information (narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	0 Months Ended
Mar. 31, 2013	Mar. 31, 2013 Princeton [Member]	Mar. 31, 2013 San Francisco [Member]	May 31, 2013 767 Fifth Avenue (The GM Building) [Member] partners
Segment Reporting Information [Line Items]
Reclassification between General and administrative expense and real estate operating expenses	$ 1.9	$ 0.4	$ 1.5
Number of Joint Venture Partners				2

The amount of a reclassification adjustment made to prior period financial statement amounts.

No definition available.

Name:	us-gaap_PriorPeriodReclassificationAdjustment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration