Accounts Receivable - Schedule of Accounts Receivable (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross accounts receivable	$ 467,722	$ 462,738
Allowance for doubtful accounts and other reserves	(1,358)	(1,281)
Accounts receivable, net	466,364	461,457
Unbilled accounts receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross accounts receivable	136,733	142,737
Trade accounts receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross accounts receivable	$ 330,989	$ 320,001