Credit Facilities (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Mature on May 31, 2016 Debt_Covenants	Sep. 30, 2014 Mature on May 31, 2017 Debt_Covenants	Sep. 30, 2014 Mature on September 15, 2019 Debt_Covenants Rate
Line of Credit Facility [Line Items]
Total of committed credit facilities with two banks	$ 325,000,000		$ 175,000,000	$ 75,000,000	$ 75,000,000
Committed credit facilities maturity			May 31, 2016	May 31, 2017	Sep. 15, 2019
Line of Credit Facility, Interest Rate					0.7533%
Interest Rate Swap Facility, Fixed Interest					2.50%
Borrowings outstanding	75,000,000	40,000,000
Stand-by letters of credit	32,700,000
Number of financial covenants for debt agreements			2	2	2
Additional borrowings of credit facilities	75,000,000
Credit facility repayments - long-term	$ 40,000,000