Revenue - Additional Information (Details)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Flex revenue \| GS \| Fixed-price Contract
Disaggregation of Revenue [Line Items]
Revenue, as a percent	41.90%	34.30%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_DisaggregationOfRevenueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration