Segment information - Revenues and long-lived assets by location of property (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets
Revenue	[1]	$ 560,999	$ 549,824	$ 551,385
Total property, plant and equipment at book value		1,168,044	1,074,676
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		136,538	128,671	132,385
Total property, plant and equipment at book value		336,263	294,773
United Kingdom
Revenues from External Customers and Long-Lived Assets
Revenue		56,432	56,016	62,656
Total property, plant and equipment at book value		52,311	48,577
United States
Revenues from External Customers and Long-Lived Assets
Revenue		112,677	108,238	107,965
Total property, plant and equipment at book value		334,634	343,342
Brazil
Revenues from External Customers and Long-Lived Assets
Revenue		59,737	73,300	69,142
Total property, plant and equipment at book value		74,944	73,843
All other countries
Revenues from External Customers and Long-Lived Assets
Revenue		195,615	183,599	$ 179,237
Total property, plant and equipment at book value		$ 369,892	$ 314,141

[1] Includes revenue from related parties of $15,449,000, $15,458,000 and $13,123,000, respectively