Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Deferred tax assets:
Federal and state tax carryforwards	$ 293,298	$ 298,426
Inventory adjustments	59,507	62,985
Warranty and other reserves	6,140	16,943
Incentive compensation	19,043	15,390
Property, equipment and other assets	3,247	2,896
Uncertain tax positions	1,332	1,721
Other	1,785	809
Total deferred tax assets	384,352	399,170
Deferred tax liabilities:
Deferred revenues	(11,297)	(22,950)
Total deferred tax liabilities	(11,297)	(22,950)
Net deferred tax assets before valuation allowance	373,055	376,220
Valuation allowance	(65,159)	(66,265)	$ (57,700)
Net deferred tax assets	$ 307,896	$ 309,955