Commitments and Contingencies Available Lines of Credit, Outstanding Standby Letter of Credits, and Bonds (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Jan. 05, 2018
Line of Credit Facility [Line Items]
Multicurrency revolving line of credit			$ 500,000
Net available for additional standby LOCs under sub-facility	$ 259,252	$ 259,017
Credit Facility [Member]
Line of Credit Facility [Line Items]
Multicurrency revolving line of credit	500,000	500,000
Long-term Line of Credit	0	0
Letters of Credit Outstanding, Amount	(40,748)	(40,983)
Unsecured Multicurrency Revolving Lines of Credit [Member]
Line of Credit Facility [Line Items]
Multicurrency revolving line of credit	107,117	108,039
Letters of Credit Outstanding, Amount	(19,300)	(19,386)
Short-term borrowings	(513)	(2,232)
Line of Credit Facility, Remaining Borrowing Capacity	87,304	86,421
Surety Bond [Member]
Line of Credit Facility [Line Items]
Unsecured surety bonds in force	111,197	94,365
Line of Credit [Member]
Line of Credit Facility [Line Items]
Long-term Line of Credit	0	0
Line of Credit Facility, Remaining Borrowing Capacity	$ 459,252	$ 459,017