Business Realignment Expenses - Narrative (Details) $ in Thousands		3 Months Ended		12 Months Ended
	Sep. 25, 2017 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 16, 2017 ft²
Restructuring Cost and Reserve [Line Items]
Payments for restructuring		$ 1,330
Restructuring charges		1,347
Restructuring reserve		3,197	$ 3,180	$ 3,180
Bethesda, Maryland | Office Building
Restructuring Cost and Reserve [Line Items]
Area of real estate property | ft²					18,412
Other Operating Expense | DISTRICT OF COLUMBIA | Office Building
Restructuring Cost and Reserve [Line Items]
Loss on contract termination				2,000
Payment on contract loss termination		300
Employee Severance
Restructuring Cost and Reserve [Line Items]
Restructuring costs			900
Payments for restructuring		944
Restructuring charges		1,225
Restructuring reserve		1,473	1,192	1,192
General and Administrative Expense
Restructuring Cost and Reserve [Line Items]
Restructuring charges		200
Selling and Marketing Expense
Restructuring Cost and Reserve [Line Items]
Restructuring charges		200
Technology and Operations
Restructuring Cost and Reserve [Line Items]
Restructuring charges		900
CAG
Restructuring Cost and Reserve [Line Items]
Restructuring costs		900
CAG | Employee Severance
Restructuring Cost and Reserve [Line Items]
Restructuring costs		500	600
Payments for restructuring		207
Restructuring charges		751
Restructuring reserve		1,337	793	793
CAG | Employee Severance | Severance Agreement
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 300
CAG | Legal and Administrative Costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs			100
IronDirect
Restructuring Cost and Reserve [Line Items]
Asset Impairment Charges				900
Reversal of earn-out liability				900
IronDirect | Contract-Based Intangible Assets
Restructuring Cost and Reserve [Line Items]
Impairment of intangible assets, finite-lived				1,200
IronDirect | Leasehold Improvements
Restructuring Cost and Reserve [Line Items]
Impairment of long-lived assets				600
IronDirect | Employee Severance
Restructuring Cost and Reserve [Line Items]
Restructuring costs				100
Corporate and Other | Employee Severance
Restructuring Cost and Reserve [Line Items]
Restructuring costs		500
Payments for restructuring		737
Restructuring charges		474
Restructuring reserve		$ 136	$ 399	$ 399