Notes and Bonds Payable (Details) (USD $) In Thousands, unless otherwise specified					12 Months Ended											12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2012 Line of Credit [Member] Unsecured Credit Facility due 2015 [Member]	Dec. 31, 2011 Line of Credit [Member] Unsecured Credit Facility due 2015 [Member]	Dec. 31, 2012 Senior Notes [Member] 5.125% Senior Notes Due 2014 [Member]	Dec. 31, 2011 Senior Notes [Member] 5.125% Senior Notes Due 2014 [Member]	Dec. 31, 2004 Senior Notes [Member] 5.125% Senior Notes Due 2014 [Member]	Dec. 31, 2012 Senior Notes [Member] 6.500% Senior Notes Due 2017 [Member]	Dec. 31, 2011 Senior Notes [Member] 6.500% Senior Notes Due 2017 [Member]	Dec. 04, 2009 Senior Notes [Member] 6.500% Senior Notes Due 2017 [Member]	Dec. 31, 2012 Senior Notes [Member] 5.750% Senior Notes Due 2021 [Member]	Dec. 31, 2011 Senior Notes [Member] 5.750% Senior Notes Due 2021 [Member]	Dec. 13, 2010 Senior Notes [Member] 5.750% Senior Notes Due 2021 [Member]	Dec. 31, 2012 Mortgage notes payable [Member] 5.000%-7.625% Mortgage Notes Payable [Member]	Dec. 31, 2011 Mortgage notes payable [Member] 5.000%-7.625% Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Notes and bonds payable	$ 1,293,044	[1]	$ 1,393,537	[1]	$ 110,000	$ 212,000	$ 264,522	$ 264,371		$ 298,728	$ 298,465		$ 397,307	$ 397,052		$ 222,487	$ 221,649
Contractual interest rates							5.125%		5.125%	6.50%		6.50%	5.75%		5.75%
Contractual interest rates, description of variable rate basis					LIBOR
Contractual interest rates, basis spread on variable rate					1.40%
Contractual interest rates, minimum																5.00%
Contractual interest rates, maximum																7.625%

[1] Level 3 - Fair value derived from valuation techniques in which one of more significant inputs or significant value drivers are unobservable.