Note 5 - Fair Value of Assets and Liabilities - Carrying Value and Estimated Fair Value of Senior Notes (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Euro Senior Notes, Series A [Member] | Reported Value Measurement [Member]
Senior Notes, Fair Value	$ 137,670	$ 122,313
Euro Senior Notes, Series A [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	137,581	122,586
Euro Senior Notes, Series B [Member] | Reported Value Measurement [Member]
Senior Notes, Fair Value	111,784	99,314
Euro Senior Notes, Series B [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	110,550	99,230
U.S. Senior Notes, Series A [Member] | Reported Value Measurement [Member]
Senior Notes, Fair Value	25,000
U.S. Senior Notes, Series A [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	24,938
U.S. Senior Notes, Series B [Member] | Reported Value Measurement [Member]
Senior Notes, Fair Value	100,000
U.S. Senior Notes, Series B [Member] | Estimate of Fair Value Measurement [Member]
Senior Notes, Fair Value	$ 99,884