Revenue from Contracts with Customers Contract Assets and Liabilities - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Trade accounts receivable, net of allowances	$ 181,454		$ 181,454		$ 144,051
Deferred Revenue	29,134		29,134		$ 17,050
Deferred Revenue, Additions			59,360
Contract with Customer, Liability, Revenue Recognized			(47,276)
Allowance for Doubtful Accounts Receivable, Write-offs	$ 300	$ 300	100	$ 900
Contract with Customer, Liability, Revenue Recognized			$ 47,300