Unsecured Line Of Credit (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 09, 2011
Line of Credit Facility [Line Items]
Unsecured term loan agreement	$ 175,000	$ 275,000
Line of credit facility expiration date	December 8, 2015
Line of credit facility new expiration date	August 31, 2012
Amount borrowed under term loan agreement	2,500
Line of credit facility effective rate	1.61%
Available amount on credit facility	264,654
LIBOR Loans [Member]
Line of Credit Facility [Line Items]
One-month LIBOR rate	0.31%
Line of credit facility effective rate	1.30%
LIBOR Loans [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Line of credit, interest rate	1.00%
LIBOR Loans [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Line of credit, interest rate	2.00%
Standby Letters Of Credit [Member]
Line of Credit Facility [Line Items]
Unsecured term loan agreement	20,000
Amount borrowed under term loan agreement	$ 7,846