Financial Instruments: Derivatives and Hedging (Details) (USD $)	1 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Financial Instruments: Derivatives and Hedging
Loss from settlement of hedges included in accumulated other comprehensive loss			$ 17,400,000
Share of joint venture accumulated other comprehensive loss			16,600,000
Net loss on forward swaps	19,500,000
Amortization period of the loss from the settlement of hedges		10 years
Accumulated other comprehensive loss relating to derivatives			34,100,000	36,200,000
Estimated current balance held in accumulated other comprehensive loss to be reclassified into interest within the next 12 months			2,100,000
Estimated amount of amount of Loss reclassified from accumulated other comprehensive loss into equity in net income from unconsolidated joint ventures within the next 12 months.			4,900,000
Amount of (Loss) or Gain Recognized in Other Comprehensive Loss (Effective Portion) on derivatives qualifying as hedges			(10,594,000)	(16,049,000)	(17,619,000)
Amount of (Loss) or Gain Reclassified from Accumulated Other Comprehensive Loss into Interest Expense/ Equity in net income of unconsolidated joint ventures (Effective Portion) on derivatives qualifying as hedges			(12,657,000)	(12,625,000)	(12,661,000)
Amount of (Loss) or Gain Recognized in Interest Expense/Equity in Net Income of Unconsolidated Joint Ventures (Ineffective Portion) on derivatives qualifying as hedges			(3,000)	(16,000)	(1,329,000)
Amount of (Loss) or Gain Recognized in Interest Expense/Equity in Net Income of Unconsolidated Joint Ventures (Ineffective Portion) on derivatives not qualifying as hedges			(847,000)	(82,000)
Interest Rate Cap expiring in April, 2013
Financial Instruments: Derivatives and Hedging
Notional Value			775,000,000
Strike Rate (as a percent)			3.65%
Interest Rate Swap Expiring in June 2016
Financial Instruments: Derivatives and Hedging
Notional Value			30,000,000
Strike Rate (as a percent)			2.295%
Interest Rate Swap expiring in February, 2015
Financial Instruments: Derivatives and Hedging
Notional Value			8,500,000
Strike Rate (as a percent)			0.74%
Interest Rate Cap expiring in March, 2013
Financial Instruments: Derivatives and Hedging
Notional Value			475,000,000
Strike Rate (as a percent)			6.00%
Interest Rate Cap expiring in November, 2013
Financial Instruments: Derivatives and Hedging
Notional Value			271,912,000
Strike Rate (as a percent)			6.00%
Level 2
Financial Instruments: Derivatives and Hedging
Fair Value			2,000,000
Level 2 | Interest Rate Swap Expiring in June 2016
Financial Instruments: Derivatives and Hedging
Fair Value			(1,881,000)
Level 2 | Interest Rate Swap expiring in February, 2015
Financial Instruments: Derivatives and Hedging
Fair Value			$ (78,000)