Notes and Bonds Payable (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Unsecured Credit Facility due 2012 [Member]
Notes and bond payable
Amount outstanding under Unsecured Credit Facility	$ 0	$ 0
Unsecured Credit Facility due 2015 [Member]
Notes and bond payable
Amount outstanding under Unsecured Credit Facility	212,000	0
Senior Notes Due 2011 [Member]
Notes and bond payable
Senior Notes	0	278,311
Senior Notes Due 2014 [Member]
Notes and bond payable
Senior Notes	264,371	264,227
Senior Notes Due 2017 [Member]
Notes and bond payable
Senior Notes	298,465	298,218
Senior Notes Due 2021 [Member]
Notes and bond payable
Senior Notes	397,052	396,812
Mortgage Notes Payable [Member]
Notes and bond payable
Mortgage notes payable, net of discounts and including premiums	$ 221,649	$ 170,287