Revenue (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accounts receivable, less allowance
Balance at January 1, 2018	$ 2,104,555	$ 1,356,851
Balance at March 31, 2018	2,326,411	2,104,555
Contract Assets (Current)
Balance at January 1, 2018	33,031
Balance at March 31, 2018	34,316	33,031
Contract Assets (Long-Term)
Balance at January 1, 2018	135,150
Balance at March 31, 2018	135,581	135,150
Contract Liabilities (Current)
Balance at January 1, 2018	208,909
Balance at March 31, 2018	153,037	208,909
Contract Liabilities (Long-Term)
Balance at January 1, 2018	8,745
Balance at March 31, 2018	$ 8,745	$ 8,745