Credit Facilities (Narrative) (Details) (USD $)	1 Months Ended			12 Months Ended
	Jan. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Mature On May 31, 2016 Debt_Covenants	Dec. 31, 2012 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
Term of credit facility (years)				4 years	5 years
Line of credit facility interest rate		0.85%
Borrowings outstanding		90,000,000	0
Credit facility repayments	20,000,000
Standby letters of credit		$ 33,800,000
Number of financial covenants for debt agreements				2	2