Notes and Bonds Payable (Details) (USD $) In Thousands, unless otherwise specified					12 Months Ended														12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2013 Line of Credit [Member] Unsecured Credit Facility due 2015 [Member]	Dec. 31, 2012 Line of Credit [Member] Unsecured Credit Facility due 2015 [Member]	Dec. 31, 2013 Senior Notes [Member] 5.125% Senior Notes Due 2014 [Member]	Apr. 18, 2013 Senior Notes [Member] 5.125% Senior Notes Due 2014 [Member]	Dec. 31, 2012 Senior Notes [Member] 5.125% Senior Notes Due 2014 [Member]	Dec. 31, 2013 Senior Notes [Member] 6.500% Senior Notes Due 2017 [Member]	Dec. 31, 2012 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, 2013 Senior Notes [Member] 5.750% Senior Notes Due 2021 [Member]	Dec. 31, 2012 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, 2013 Senior Notes [Member] 3.75% Senior Notes Due 2023 [Member]	Mar. 26, 2013 Senior Notes [Member] 3.75% Senior Notes Due 2023 [Member]	Dec. 31, 2012 Senior Notes [Member] 3.75% Senior Notes Due 2023 [Member]	Dec. 31, 2013 Mortgage notes payable [Member]	Dec. 31, 2013 Mortgage notes payable [Member] 5.000%-7.625% Mortgage Notes Payable [Member]	Dec. 31, 2012 Mortgage notes payable [Member] 5.000%-7.625% Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Notes and bonds payable	$ 1,348,459	[1]	$ 1,293,044	[1]	$ 238,000	$ 110,000	$ 0		$ 264,522	$ 299,008	$ 298,728		$ 397,578	$ 397,307		$ 248,077		$ 0		$ 165,796	$ 222,487
Contractual interest rates							5.125%	5.125%		6.50%		6.50%	5.75%		5.75%	3.75%	3.75%
Contractual interest rates, description of variable rate basis					LIBOR
Contractual interest rates, basis spread on variable rate					1.40%	1.40%
Contractual interest rates, minimum																			5.00%
Contractual interest rates, maximum																			7.63%

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