Income Taxes (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Sep. 30, 2017	Oct. 01, 2016	Sep. 30, 2017	Oct. 01, 2016
Income Taxes
Income tax benefit (provision)		$ (2,178)	$ (1,344)	$ (1,469)	$ (3,319)
Effective income tax rate reconciliation
Effective income tax rate (as a percent)		9.80%	6.30%	2.80%	7.40%
Components of deferred taxes
Gross unrecognized tax benefits		$ 3,300		$ 3,300
Gross unrecognized tax benefits which would affect the effective tax rate if recognized		1,800		1,800
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations		500
Amount of estimated decrease in unrecognized tax benefits		$ 1,900		$ 1,900
Norwegian | 2013
Income Taxes
Adjustments to taxable income excluding interest and penalties	$ 33,000