Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2016	Mar. 31, 2018	Apr. 01, 2017	Mar. 31, 2018	Apr. 01, 2017	Oct. 01, 2017	Sep. 30, 2017
Income Tax Disclosure [Line Items]
Effective income tax rate		31.00%	(5.00%)	(21.00%)	(36.00%)
Income before income taxes		$ 11,546	$ (1,056)	$ 18,017	$ (7,559)
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent				21.00%	35.00%
Operating Loss Carryforwards, Limitations on Use				0.8
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount		(7,000)	$ (4,000)	$ (9,000)	$ (12,000)
Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Percent				24.50%
Unrecognized tax benefits		14,300		$ 14,300			$ 14,800
Income tax provision upon recognition of unrecognized tax benefit		14,300		14,300
Unrecognized tax benefits that would impact valuation allowance		3,900		3,900
Potentail reduction in unrecognized tax benefits and accrued interest over next 12 months		$ (7,000)		(7,000)
Korea
Income Tax Disclosure [Line Items]
Income tax examination, estimate of possible loss	$ 12,000
US Tax on Undistributed Foreign Earnings [Member]
Income Tax Disclosure [Line Items]
Current Federal, State and Local, Tax Expense (Benefit)				7,100
Net Decrease in Deferred Tax Liability [Member]
Income Tax Disclosure [Line Items]
Deferred Income Tax Expense (Benefit)				$ 14,100
Accounting Standards Update 2016-09 [Member]
Income Tax Disclosure [Line Items]
Windfall Tax Deductions Not Yet Recognized						$ 37,000
Windfall Tax Deductions Not Yet Recognized Offset Valuation Allowance						$ 36,900