Fair Value Measurements (Fair value measurements on a recurring basis) (Details) - Fair value, measurements, recurring [Member] - USD ($) $ in Thousands	Jul. 01, 2018	Dec. 31, 2017
Fair value, inputs, level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	$ 20,248	$ 27,766
Liabilities, fair value disclosure	0	0
Fair value, inputs, level 1 [Member] | Other current assets, net [Member] | Interest rate swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments - interest rate swaps, current assets	0	0
Fair value, inputs, level 1 [Member] | Other assets, net [Member] | Interest rate swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments - interest rate swaps, non-current assets	0	0
Fair value, inputs, level 1 [Member] | Accrued and other current liabilities [Member] | Interest rate swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments - interest rate swaps, current liabilities	0	0
Fair value, inputs, level 1 [Member] | Fixed income funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value disclosure	5,360	1,830
Fair value, inputs, level 1 [Member] | Money market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value disclosure	10,367	24,656
Restricted cash equivalents, fair value disclosure	4,521	1,280
Fair value, inputs, level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	1,162	67
Liabilities, fair value disclosure	0	1,010
Fair value, inputs, level 2 [Member] | Other current assets, net [Member] | Interest rate swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments - interest rate swaps, current assets	1,162	0
Fair value, inputs, level 2 [Member] | Other assets, net [Member] | Interest rate swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments - interest rate swaps, non-current assets	0	67
Fair value, inputs, level 2 [Member] | Accrued and other current liabilities [Member] | Interest rate swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments - interest rate swaps, current liabilities	0	1,010
Fair value, inputs, level 2 [Member] | Fixed income funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value disclosure	0	0
Fair value, inputs, level 2 [Member] | Money market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value disclosure	0	0
Restricted cash equivalents, fair value disclosure	0	0
Reported value measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	21,410	27,833
Liabilities, fair value disclosure	0	1,010
Reported value measurement [Member] | Other current assets, net [Member] | Interest rate swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments - interest rate swaps, current assets	1,162	0
Reported value measurement [Member] | Other assets, net [Member] | Interest rate swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments - interest rate swaps, non-current assets	0	67
Reported value measurement [Member] | Accrued and other current liabilities [Member] | Interest rate swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments - interest rate swaps, current liabilities	0	1,010
Reported value measurement [Member] | Fixed income funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value disclosure	5,360	1,830
Reported value measurement [Member] | Money market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value disclosure	10,367	24,656
Restricted cash equivalents, fair value disclosure	$ 4,521	$ 1,280