Commitments and Contingencies Available Lines of Credit, Outstanding Standby Letter of Credits, and Bonds (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 05, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Multicurrency revolving line of credit		$ 500,000
Line of Credit Facility, Capacity remaining to support letters of credit	$ 253,641		$ 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	95,777		125,414
Letters of Credit Outstanding, Amount	(46,359)		(31,881)
Unsecured Multicurrency Revolving Lines of Credit [Member]
Line of Credit Facility [Line Items]
Multicurrency revolving line of credit	114,160		110,477
Letters of Credit Outstanding, Amount	(21,471)		(21,030)
Short-term borrowings	(1,776)		(916)
Line of Credit Facility, Remaining Borrowing Capacity	90,913		88,531
Surety Bond [Member]
Line of Credit Facility [Line Items]
Unsecured surety bonds in force	52,639		51,344
Line of Credit [Member]
Line of Credit Facility [Line Items]
Long-term Line of Credit	95,777		125,414
Line of Credit Facility, Remaining Borrowing Capacity	$ 357,864		$ 342,705