Pensions - Schedule of fair value of plan assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure
Fair value of plan assets	$ 25,564	$ 28,232	$ 24,018	$ 24,202
Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	23,672	26,053
Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets	0	0
Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	1,892	2,179
Cash
Defined Benefit Plan Disclosure
Fair value of plan assets	733	1,908
Cash | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	733	1,908
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	3,800	4,529
U.K. managed funds | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	3,800	4,529
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	7,437	8,486
Overseas managed funds | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	7,437	8,486
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	3,701	3,058
U.K. government bonds | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	3,701	3,058
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	3,910	3,437
Corporate bonds | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	3,910	3,437
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	4,091	4,635
Quoted hedge funds | Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	4,091	4,635
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	1,892	2,179
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	$ 1,892	$ 2,179	$ 1,942