Note 4 - Debt (Details Textual) $ in Thousands, € in Millions, ¥ in Millions					25 Months Ended	30 Months Ended
	Feb. 15, 2017 USD ($)	Dec. 08, 2016 USD ($)	Dec. 08, 2016 EUR (€)	Mar. 04, 2016 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)	Oct. 13, 2017 USD ($)	Oct. 12, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 27, 2014 USD ($)	Dec. 27, 2014 CNY (¥)
Entrusted Loan										$ 3,522
Euro Senior Notes, Series A and B [Member]
Proceeds from Notes Payable | €			€ 212
Euro Senior Notes, Series A [Member]
Proceeds from Notes Payable | €			€ 117
Debt Instrument, Interest Rate, Stated Percentage		1.14%	1.14%
Euro Senior Notes, Series B [Member]
Proceeds from Notes Payable | €			€ 95
Debt Instrument, Interest Rate, Stated Percentage		1.83%	1.83%
U.S. Senior Notes, Series A and B [Member]
Proceeds from Notes Payable		$ 125,000
U.S. Senior Notes, Series A [Member]
Proceeds from Notes Payable	$ 25,000
Debt Instrument, Interest Rate, Stated Percentage	3.03%
U.S. Senior Notes, Series B [Member]
Proceeds from Notes Payable	$ 100,000
Debt Instrument, Interest Rate, Stated Percentage	3.74%
Entrustment Loan Member
Debt Instrument, Interest Rate, Stated Percentage											5.25%	5.25%
Entrusted Loan											$ 17,900	¥ 110
Credit Agreement [Member]
Line of Credit Facility, Maximum Borrowing Capacity				$ 700,000
Line of Credit Facility, Additional Uncommitted Borrowing Capacity				$ 150,000			$ 300,000
Number of Advances							2
Second Advance, Maximum Amount							$ 75,000
Line of Credit Facility, Expiration Period				5 years
Line of Credit Facility, Additional Uncommitted Borrowing Capacity, Minimum Increments				$ 25,000
Debt Instrument, Interest Rate, Effective Percentage									2.73%
Letters of Credit Outstanding, Amount									$ 100
Credit Agreement [Member] | Minimum [Member]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage				0.15%
Credit Agreement [Member] | Maximum [Member]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage				0.30%
Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Debt Instrument, Basis Spread on Variable Rate				1.00%
Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]
Debt Instrument, Basis Spread on Variable Rate				2.00%
Credit Agreement [Member] | Base Rate [Member] | Minimum [Member]
Debt Instrument, Basis Spread on Variable Rate				0.00%
Credit Agreement [Member] | Base Rate [Member] | Maximum [Member]
Debt Instrument, Basis Spread on Variable Rate				1.00%
Credit Agreement [Member] | Revolving Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity				$ 575,000			700,000	$ 575,000
Line of Credit Facility, Remaining Borrowing Capacity									$ 559,900
Credit Agreement [Member] | Term Loan Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity				$ 125,000			$ 200,000	$ 125,000
Credit Agreement [Member] | Term Loan Credit Facility [Member] | Scenario, Forecast [Member]
Line of Credit Facility, Periodic Payment, Principal					$ 1,600	$ 3,100