Financial Instruments - Credit Risks (Details) - USD ($) $ in Thousands |
3 Months Ended | 12 Months Ended |
---|---|---|
Mar. 31, 2018 |
Dec. 31, 2017 |
|
Concentration Risk [Line Items] | ||
Allowance for doubtful accounts receivable, period increase (decrease) | $ 3,200 | $ 9,200 |
Credit Concentration Risk | ||
Concentration Risk [Line Items] | ||
Maximum exposure | 979,738 | 1,002,410 |
Carrying Value | Cash and cash equivalents | ||
Concentration Risk [Line Items] | ||
Maximum exposure | 483,874 | 414,111 |
Carrying Value | Trade receivables, net of allowances | ||
Concentration Risk [Line Items] | ||
Maximum exposure | 395,707 | 484,101 |
Carrying Value | Other taxes | ||
Concentration Risk [Line Items] | ||
Maximum exposure | 52,557 | 58,346 |
Carrying Value | Other current assets | ||
Concentration Risk [Line Items] | ||
Maximum exposure | 26,463 | 26,327 |
Carrying Value | Non-current financial assets | ||
Concentration Risk [Line Items] | ||
Maximum exposure | $ 21,137 | $ 19,525 |