Restructuring Costs - Summary of Pre-tax Costs Incurred (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Restructuring Cost And Reserve [Line Items]
Restructuring costs	$ 0	$ 2	$ 33	$ 147
Employee Severance
Restructuring Cost And Reserve [Line Items]
Restructuring costs	0	0	33	214
Contract Termination And Other Costs
Restructuring Cost And Reserve [Line Items]
Restructuring costs	$ 0	$ 2	$ 0	$ (67)

Amount, after cash payment, of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Name:	us-gaap_RestructuringCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration