Fair Value of Assets and Liabilities (Details 2) (USD $) In Thousands, except Per Share data, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012 GOV	Dec. 31, 2011 GOV	Jun. 30, 2012 Carrying Amount	Dec. 31, 2011 Carrying Amount	Jun. 30, 2012 Carrying Amount GOV	Dec. 31, 2011 Carrying Amount GOV	Jun. 30, 2012 Total Fair value	Dec. 31, 2011 Total Fair value	Jun. 30, 2012 Total Fair value GOV	Dec. 31, 2011 Total Fair value GOV
Fair value of financial instruments
Equity investments						$ 169,421	$ 172,186			$ 225,069	$ 224,373
Senior notes and mortgage notes payable				$ 2,764,861	$ 2,745,331			$ 2,916,469	$ 2,924,141
Quoted market price (in dollars per share)		$ 22.62	$ 22.55
Concentration risk, percentage	3.00%