Credit Facilities (Narrative) (Details) (USD $)			6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Mature On May 31, 2016 Debt_Covenants	Jun. 30, 2013 Mature On May 31, 2017 Debt_Covenants
Line of Credit Facility [Line Items]
Total of committed credit facilities with two banks	$ 250,000,000		$ 175,000,000	$ 75,000,000
Committed credit facilities maturity			May 31, 2016	May 31, 2017
Line of Credit Facility, Interest Rate at Period End	0.85%
Term of credit facility (years)			4 years	5 years
Borrowings outstanding	40,000,000	0
Stand-by letters of credit	$ 33,700,000
Number of financial covenants for debt agreements			2	2