Tax Credit Transactions (Details) (USD $)	12 Months Ended							12 Months Ended	0 Months Ended	1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Historic Tax Credit Transaction [Member]	Dec. 31, 2011 Historic Tax Credit Transaction [Member]	Dec. 31, 2010 Historic Tax Credit Transaction [Member]	Nov. 17, 2008 Historic Tax Credit Transaction [Member] sqft	Dec. 31, 2012 New Markets Tax Credit Transaction [Member]	Oct. 19, 2011 USB [Member] Historic Tax Credit Transaction [Member]	Jun. 30, 2010 USB [Member] Historic Tax Credit Transaction [Member]	Dec. 31, 2009 USB [Member] Historic Tax Credit Transaction [Member]	Nov. 30, 2008 USB [Member] Historic Tax Credit Transaction [Member]	Dec. 31, 2012 USB [Member] Historic Tax Credit Transaction [Member]	Dec. 31, 2011 USB [Member] Historic Tax Credit Transaction [Member]	Aug. 27, 2010 USB [Member] Historic Tax Credit Transaction [Member]	Dec. 31, 2011 USB [Member] New Markets Tax Credit Transaction [Member]
Tax Credit Transactions (Textuals) [Abstract]
Area of IRS Philadelphia Campus building (in square foot)							862,692
Percentage of building leased to IRS							100.00%
Agreed contribution in project cost by USB															$ 64,100,000
Amount received in advance for project cost	0	2,694,000	27,396,000							27,400,000	23,800,000	10,200,000
Amount received from contribution									2,700,000
Tax credit conditions				20% recapture per year beginning one year after the completion of the IRS Philadelphia Campus				100% recapture for a period of seven years as provided in the Internal Revenue Code
Percentage of tax credit recapture	100.00%			20.00%
USB contributions presented within deferred income								13,300,000					39,100,000	51,600,000		13,300,000
Contribution allocated in non controlling interest				2,600,000	2,400,000
Recognition of cash received as revenue net of allocated expenses on or after September 2011, period				over the five year credit recapture period as defined in the Internal Revenue Code
Cash received as revenue				11,800,000	12,000,000
Allocated expenses within other income (expense)				500,000	500,000
Deferred transaction costs	122,243,000	115,362,000		1,600,000	2,100,000			5,300,000								5,300,000
Rate of return on noncontrolling interest expected				2.00%
Accretion of non-controlling interest liability				1,300,000	1,300,000	1,100,000
Expected Benefit From a Qualified NMTC Program								$ 7,800,000
Limit on tax credit claims under the Community Renewal Tax Relief	up to 39% of qualified investments in qualified, active low-income businesses or ventures
Percentage of qualified investments	39.00%
Recognition of cash received as revenue net of allocated expenses on or after September 2011, period in years				5 years				7 years