Revenue from Contracts with Customers Contract Assets and Liabilities - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Trade accounts receivable, net of allowances	$ 175,262		$ 144,051
Deferred Revenue	35,929		$ 17,050
Allowance for Doubtful Accounts Receivable, Write-offs	200	$ 600
Contract with Customer, Liability, Revenue Recognized	17,800
Total Change in Deferred Revenue	$ 18,879