Summary of Significant Accounting Policies - Supplemental Cash Flow Items (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2018	Jul. 01, 2017
Supplemental Disclosures of Cash Flow Information
Interest paid	$ 5,202	$ 3,102
Taxes paid	91	685
Non-cash activities:
Purchases of property and equipment not paid	$ 1,589	$ 2,878

Future cash outflow to pay for purchases of fixed assets that have occurred.

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