Accrued Liabilities - Carrying Value of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities and Other Liabilities [Abstract]
Current deferred revenue	$ 101,194	$ 90,639
Wages, severance and related taxes	56,129	57,633
Accrued rebates	41,312	38,025
Employee benefits	25,670	25,406
Accrued interest	18,530	22,019
Other (individual items less than 5% of total current liabilities)	121,441	68,929
Accrued liabilities	$ 364,276	$ 302,651