Pensions - Schedule of fair value of plan assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 24,018	$ 24,202	$ 24,968	$ 23,162
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22,076	22,176
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,942	2,026
Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,840	1,101
Cash | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,840	1,101
Cash | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.K. managed funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,461	5,509
U.K. managed funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,461	5,509
U.K. managed funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.K. managed funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Overseas managed funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,667	4,999
Overseas managed funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,667	4,999
Overseas managed funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Overseas managed funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.K. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,774	860
U.K. government bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,774	860
U.K. government bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.K. government bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,004	5,847
Corporate bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,004	5,847
Corporate bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Quoted hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,330	3,860
Quoted hedge funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,330	3,860
Quoted hedge funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Quoted hedge funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Annuities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,942	2,026
Annuities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Annuities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Annuities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,942	$ 2,026	$ 2,186