Retirement Plans - UK plans (Details) - Defined benefit plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Accrued pension liability	$ 38,578	$ 42,786	$ 42,763
Fair value of plan assets	85,848	77,252	79,402		$ 49,281
Funding deficit based on most recent valuation	38,578	42,786	$ 42,763
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Accrued pension liability	$ 2,300	$ 4,100
Expected long-term rate of return on plan assets	2.57%	2.77%
Fair value of plan assets	$ 43,161	$ 38,520
Funding deficit based on most recent valuation	$ (1,606)	$ 2,316		$ 1,000
Period over which the deficit is being funded	4 years
United Kingdom | Equities
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets	5.60%	5.70%
Fair value of plan assets	$ 11,035	$ 9,122
United Kingdom | Gilts
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets	1.90%	2.40%
Fair value of plan assets	$ 15,785	$ 15,459
United Kingdom | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets	2.60%	3.30%
Fair value of plan assets	$ 16,021	$ 13,850
United Kingdom | Other (cash)
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets	0.50%	0.50%
Fair value of plan assets	$ 320	$ 89