Receivables - Schedule of Receivables (Details) - USD ($) $ in Thousands |
Dec. 31, 2017 |
Dec. 31, 2016 |
|---|---|---|
| Receivables [Abstract] | ||
| Billed receivables | $ 50,595 | $ 41,992 |
| Unbilled contract receivables | 121,601 | 127,150 |
| Other | 2,799 | 2,930 |
| Receivables | 174,995 | $ 172,072 |
| Estimated unbilled contract receivables related to requests for equitable adjustments and contract line item realignments | $ 6,700 |