Revenue from Contracts with Customers - Unbilled contracts (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Deferred Revenue Arrangement [Line Items]
Contracts expected to be billed remainder of year	$ 51,749
Unbilled Receivables, Expected To Be Billed In 2019	26,802
Unbilled Receivables, Expected To Be Billed In 2020	11,801
Unbilled Receivables, Expected To Be Billed In 2021	1,150
Unbilled Receivables, Expected To Be Billed In 2022	357
Unbilled Receivables, Expected To Be Billed Thereafter	20
Unbilled Receivables, Not Billable	91,879
ASC 606
Deferred Revenue Arrangement [Line Items]
Contracts expected to be billed remainder of year	46,682
Unbilled Receivables, Expected To Be Billed In 2019	24,608
Unbilled Receivables, Expected To Be Billed In 2020	10,730
Unbilled Receivables, Expected To Be Billed In 2021	1,111
Unbilled Receivables, Expected To Be Billed In 2022	348
Unbilled Receivables, Expected To Be Billed Thereafter	20
Unbilled Receivables, Not Billable	83,499
Operating Lease Expected Revenue
Deferred Revenue Arrangement [Line Items]
Contracts expected to be billed remainder of year	5,067
Unbilled Receivables, Expected To Be Billed In 2019	2,194
Unbilled Receivables, Expected To Be Billed In 2020	1,071
Unbilled Receivables, Expected To Be Billed In 2021	39
Unbilled Receivables, Expected To Be Billed In 2022	9
Unbilled Receivables, Expected To Be Billed Thereafter	0
Unbilled Receivables, Not Billable	$ 8,380