Notes Payable (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011
Components of Notes Payable
Notes Payable	$ 302,000	$ 314,193	$ 308,000	$ 292,118	$ 296,500	$ 289,573	$ 270,500	$ 265,771
Unsecured Revolving Lines of Credit [Member]
Components of Notes Payable
Notes Payable	202,000				196,500
4.03% Senior Notes due in 2018 [Member]
Components of Notes Payable
Notes Payable	$ 100,000				$ 100,000