Fair value measurements Book and Fair Values (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Available-for-sale securities	$ 4,469	$ 2,709
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 securities	4,469	2,709
Interest rate swap agreements	2,870	0
Interest rate swap agreements	6,191	20,661
Secured notes payable	708,831	716,144
Unsecured senior notes payable	1,048,230	549,805
Unsecured senior line of credit	204,000	566,000
Unsecured senior bank term loans	1,100,000	1,350,000
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 securities	4,469	2,709
Interest rate swap agreements	2,870	0
Interest rate swap agreements	6,191	20,661
Secured notes payable	736,772	788,455
Unsecured senior notes payable	1,043,125	593,350
Unsecured senior line of credit	193,714	567,196
Unsecured senior bank term loans	$ 1,099,897	$ 1,405,124