Segment information - Revenues by geography (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	[1]	$ 165,865	$ 154,490	$ 260,732	$ 251,892
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue		0	0	0	0
Italy
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue		48,251	45,939	49,618	48,182
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue		18,396	20,820	24,483	27,618
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue		32,219	30,188	58,282	54,283
Brazil
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue		11,753	13,764	29,916	32,207
All other countries
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue		$ 55,246	$ 43,779	$ 98,433	$ 89,602

[1] Includes revenue from related parties of $4,486,000, $3,609,000, $6843,000 and $6,306,000 respectively