Segment information - Revenues and long-lived assets by location of property (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets
Revenue	[1]	$ 576,836	$ 560,999	$ 549,824
Total property, plant and equipment at book value		1,261,932	1,168,044
Bermuda
Revenues from External Customers and Long-Lived Assets
Revenue		0	0	0
Total property, plant and equipment at book value		0	0
Italy
Revenues from External Customers and Long-Lived Assets
Revenue		186,232	136,538	128,671
Total property, plant and equipment at book value		366,223	336,263
United Kingdom
Revenues from External Customers and Long-Lived Assets
Revenue		40,853	56,432	56,016
Total property, plant and equipment at book value		43,783	52,311
United States
Revenues from External Customers and Long-Lived Assets
Revenue		110,209	112,677	108,238
Total property, plant and equipment at book value		326,840	334,634
Brazil
Revenues from External Customers and Long-Lived Assets
Revenue		59,473	59,737	73,300
Total property, plant and equipment at book value		62,734	74,944
All other countries
Revenues from External Customers and Long-Lived Assets
Revenue		180,069	195,615	$ 183,599
Total property, plant and equipment at book value		$ 462,352	$ 369,892

[1] Includes revenue from related parties of $15,783,000, $15,668,000 and $15,458,000, respectively