Senior Credit Facility (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Line of Credit Facility [Line Items]
Credit facility expiration date, years	five
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