Commitments and Contingencies Available Lines of Credit, Outstanding Standby Letter of Credits, and Bonds (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 05, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Multicurrency revolving line of credit		$ 500,000
Net available for additional standby LOCs under sub-facility	$ 258,892		$ 218,119
Credit Facility [Member]
Line of Credit Facility [Line Items]
Multicurrency revolving line of credit	500,000		500,000
Long-term Line of Credit	(10,000)		(125,414)
Letters of Credit Outstanding, Amount	(41,108)		(31,881)
Unsecured Multicurrency Revolving Lines of Credit [Member]
Line of Credit Facility [Line Items]
Multicurrency revolving line of credit	106,281		110,477
Letters of Credit Outstanding, Amount	(19,577)		(21,030)
Short-term borrowings	(180)		(916)
Line of Credit Facility, Remaining Borrowing Capacity	86,524		88,531
Surety Bond [Member]
Line of Credit Facility [Line Items]
Unsecured surety bonds in force	96,157		51,344
Line of Credit [Member]
Line of Credit Facility [Line Items]
Long-term Line of Credit	(10,000)		(125,414)
Line of Credit Facility, Remaining Borrowing Capacity	$ 448,892		$ 342,705