Credit Facilities (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Total of committed credit facilities with banks		$ 325,000
Credit facility repayments		20,000	$ 10,000	$ 10,000
Borrowings outstanding		180,000	$ 75,000
Standby letters of credit		25,800
Mature on July 12, 2020
Line of Credit Facility [Line Items]
Total of committed credit facilities with banks		$ 100,000
Committed credit facilities maturity		Jul. 12, 2020
Mature on September 15, 2019
Line of Credit Facility [Line Items]
Total of committed credit facilities with banks		$ 75,000
Committed credit facilities maturity		Sep. 15, 2019
Line of credit facility interest rate		1.30%
Interest rate swap facility, fixed interest		2.50%
Borrowings outstanding		$ 75,000
Mature on July 13, 2020
Line of Credit Facility [Line Items]
Total of committed credit facilities with banks		$ 75,000
Committed credit facilities maturity		Jul. 13, 2020
Mature on March 5, 2020
Line of Credit Facility [Line Items]
Total of committed credit facilities with banks		$ 75,000
Committed credit facilities maturity		Mar. 05, 2020
Additional Outstanding Debt Under Credit Facilities [Member]
Line of Credit Facility [Line Items]
Line of credit facility interest rate		1.30%
Borrowings outstanding		$ 105,000
Subsequent Event | Mature on July 12, 2020
Line of Credit Facility [Line Items]
Credit facility repayments	$ 20,000