Fair value measurements Book and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
“Available-for-sale” marketable equity securities	$ 135,267	$ 74,265
Interest Rate Derivative Liabilities, at Fair Value	8,679	909
Book Value
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
“Available-for-sale” marketable equity securities	135,267	74,265
Interest Rate Derivative Liabilities, at Fair Value	8,679	909
Secured notes payable	773,619	652,209
Senior Notes Fair Value Disclosure	1,747,613	1,747,370
Unsecured senior line of credit	843,000	304,000
Unsecured Bank Term Loans Fair Value Disclosure	950,000	975,000
Fair Value
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
“Available-for-sale” marketable equity securities	135,267	74,265
Interest Rate Derivative Liabilities, at Fair Value	8,679
Secured notes payable	807,566	693,338
Senior Notes Fair Value Disclosure	1,776,375	1,793,255
Unsecured senior line of credit	843,909	304,369
Unsecured Bank Term Loans Fair Value Disclosure	$ 951,540	$ 976,010