Reconciliation of Benefit Obligations and Plan Assets of U.K. Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation:
Projected benefit obligation at beginning of year	$ 37,261	$ 35,914
Interest cost	991	1,232	$ 1,340
Actuarial (gains) losses	(27)	7,425
Benefits paid	(1,313)	(809)
Foreign currency exchange rate changes	3,417	(6,501)
Projected benefit obligation at end of year	40,329	37,261	35,914
Accumulated benefit obligation at end of year	40,329	37,261
Change in plan assets:
Fair value of plan assets at beginning of year	31,304	32,374
Actual return on plan assets	2,605	4,522
Employer contributions	887	868
Benefits paid	(1,313)	(809)
Foreign currency exchange rate changes	2,993	(5,651)
Fair value of plan assets at end of year	36,476	31,304	32,374
Funded status at end of year	(3,853)	(5,957)
Amounts included in accumulated other comprehensive loss not yet recognized in net periodic pension cost:
Net actuarial losses at beginning of year	(11,697)	(9,874)
Net actuarial gains (losses) during the year	967	(4,469)
Amounts reclassified from accumulated other comprehensive income to income before income taxes	1,045	726
Foreign currency exchange rate changes	(808)	1,920
Net actuarial loss	(10,493)	(11,697)	$ (9,874)
Amounts expected to be amortized from accumulated other comprehensive loss into net periodic pension cost over the next fiscal year consists of:
Net actuarial loss	$ 957	$ 1,148
Defined Benefit Plan, Sponsor Location [Extensible List]	us-gaap:ForeignPlanMember	us-gaap:ForeignPlanMember	us-gaap:ForeignPlanMember