Pensions - Schedule of fair value of plan assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure
Fair value of plan assets	$ 28,232	$ 24,018	$ 24,202	$ 24,968
Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	26,053	22,076
Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	2,179	1,942
Cash
Defined Benefit Plan Disclosure
Fair value of plan assets	1,908	2,840
Cash | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	1,908	2,840
Cash | Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Cash | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
U.K. managed funds
Defined Benefit Plan Disclosure
Fair value of plan assets	4,529	4,461
U.K. managed funds | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	4,529	4,461
U.K. managed funds | Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
U.K. managed funds | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Overseas managed funds
Defined Benefit Plan Disclosure
Fair value of plan assets	8,486	5,667
Overseas managed funds | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	8,486	5,667
Overseas managed funds | Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Overseas managed funds | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
U.K. government bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	3,058	2,774
U.K. government bonds | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	3,058	2,774
U.K. government bonds | Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
U.K. government bonds | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Corporate bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	3,437	2,004
Corporate bonds | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	3,437	2,004
Corporate bonds | Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Corporate bonds | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Quoted hedge funds
Defined Benefit Plan Disclosure
Fair value of plan assets	4,635	4,330
Quoted hedge funds | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	4,635	4,330
Quoted hedge funds | Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Quoted hedge funds | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Annuities
Defined Benefit Plan Disclosure
Fair value of plan assets	2,179	1,942
Annuities | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Annuities | Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Annuities | Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	$ 2,179	$ 1,942	$ 2,026