Segment Information Segment Information (narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		0 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	May 31, 2013 767 Fifth Avenue (The GM Building) [Member] partners	Sep. 30, 2013 Princeton [Member]	Sep. 30, 2012 Princeton [Member]	Sep. 30, 2013 Princeton [Member]	Sep. 30, 2012 Princeton [Member]	Sep. 30, 2013 San Francisco [Member]	Sep. 30, 2012 San Francisco [Member]	Sep. 30, 2013 San Francisco [Member]	Sep. 30, 2012 San Francisco [Member]
Segment Reporting Information [Line Items]
Reclassification between general and administrative expense and real estate operating expense	$ 2.0	$ 1.9	$ 6.1	$ 5.8		$ 0.3	$ 0.4	$ 1.3	$ 1.4	$ 1.7	$ 1.5	$ 4.8	$ 4.4
Number of Joint Venture Partners					2