Credit Facilities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012 Debt_Covenants
Line of Credit Facility [Line Items]
Total of committed credit facilities with two banks	$ 250
Borrowings outstanding	0
Stand-by letters of credit	33.8
Mature On May 31, 2016
Line of Credit Facility [Line Items]
Total of committed credit facilities with two banks	175
Committed credit facilities maturity	May 31, 2016
Term of credit facility (years)	4 years
Number of financial covenants for debt agreements	2
Mature On May 31, 2017
Line of Credit Facility [Line Items]
Total of committed credit facilities with two banks	$ 75
Committed credit facilities maturity	May 31, 2017
Term of credit facility (years)	5 years
Number of financial covenants for debt agreements	2