Income Taxes - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 30, 2016
Income Tax Examination [Line Items]
Reported effective tax rate									28.30%	39.40%	35.80%
Deferred tax asset reduction									$ (13,900)
Change in valuation allowance									(3,200)	$ (13,900)
Increase (decrease) in deferred tax assets And Valuation Allowance									(600)
Valuation allowance	$ 34,776								31,597	34,776
Gross deferred tax assets	42,064								39,188	42,064
Credit to tax expense	252	$ (597)	$ (189)	$ (787)	$ (526)	$ (930)	$ (879)	$ (627)	(1,321)	(2,962)	$ (1,242)
Loss carry-forwards	17,161								17,109	17,161
U.S. federal
Income Tax Examination [Line Items]
Loss carry-forwards									81,500			$ 87,300
State
Income Tax Examination [Line Items]
Loss carry-forwards									70,900
Foreign subsidiary
Income Tax Examination [Line Items]
Gross deferred tax assets	$ 900				$ 8,400				6,600	900	$ 8,400
Credit to tax expense									200	$ 1,700
UK and US Subsidiary
Income Tax Examination [Line Items]
Credit to tax expense									(1,200)
2026 Through 2035
Income Tax Examination [Line Items]
Tax Credit Carryforward, Amount									80,900
No expiration
Income Tax Examination [Line Items]
Tax Credit Carryforward, Amount									$ 600