Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Deferred tax assets:
Federal and state tax carryforwards	$ 196,702	$ 293,298
Inventory adjustments	29,565	59,507
Incentive compensation	11,959	19,043
Warranty and other reserves	6,350	6,140
Property, equipment and other assets	2,123	3,247
Other	734	1,785
Uncertain tax positions	734	1,332
Total deferred tax assets	248,167	384,352
Deferred tax liabilities:
Deferred revenues	0	(11,297)
Total deferred tax liabilities	0	(11,297)
Net deferred tax assets before valuation allowance	248,167	373,055
Valuation allowance	(34,212)	(65,159)
Net deferred tax assets	$ 213,955	$ 307,896