Summary of Significant Accounting Policies - Recently Issued Accounting Standards (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 28, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 31, 2017	Sep. 29, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative-effect adjustment to retained earnings	$ 82,548				$ 85,029				$ 82,548	$ 85,029
Revenue	93,129	$ 90,260	$ 92,667	$ 82,713	$ 74,644	$ 74,039	$ 75,521	$ 77,006	358,769	301,210	$ 324,893
ASU 2018-02 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Amount reclassified to retained earnings	$ 300								$ 300
Sales Revenue [Member] | Gross Basis [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue, percent									1.30%
ASU 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue									$ 4,600
ASU 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Maximum [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative-effect adjustment to retained earnings												$ 100
ASU 2014-09 [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue									$ 354,200
ASU 2017-07 [Member] | Interest And Other Expenses [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Impact of change in accounting principle										200	400
ASU 2017-07 [Member] | Direct Costs [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Impact of change in accounting principle										100
ASU 2017-07 [Member] | Selling General and Administrative Expenses [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Impact of change in accounting principle										$ (300)	$ (400)