Revenue Recognition - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018
Contracts with Customers, Asset and Liability [Line Items]
Advance billings on contracts	$ 77,140	$ 73,390
Contracts in Progress
Contracts with Customers, Asset and Liability [Line Items]
Contract with customer, asset	325,745	250,325
Accounts Receivable
Contracts with Customers, Asset and Liability [Line Items]
Contract with customer, asset	93,258	82,801
Other Assets
Contracts with Customers, Asset and Liability [Line Items]
Contract with customer, asset	$ 1,656	$ 1,669