Senior Credit Facilities (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Line of Credit Facility [Line Items]
Unsecured credit facility	$ 75,000
Credit facility expiration date, years	five
Credit facility outstanding amount	0	0
Credit facility expenses	500
Interest expense	2,676	2,905	8,013
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Available additional borrowing capacity	150,000
Five-Year Term Loan Facility [Member]
Line of Credit Facility [Line Items]
Credit facility initial aggregate principal amount	350,000
Effective interest rate	2.80%
Five-Year Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility initial aggregate principal amount	100,000
Unsecured Credit Facility [Member]
Line of Credit Facility [Line Items]
Interest expense	$ 2,676	$ 2,905	$ 8,013