Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Deferred tax assets:
Federal and state tax carryforwards	$ 298,426	$ 292,346
Inventory adjustments	62,985	87,335
Warranty and other reserves	16,943	14,913
Incentive compensation	15,390	10,780
Property, equipment and other assets	2,896	2,866
Uncertain tax positions	1,721	1,917
Other	809	3,814
Total deferred tax assets	399,170	413,971
Deferred tax liabilities:
Deferred revenues	(22,950)	(30,939)
Total deferred tax liabilities	(22,950)	(30,939)
Net deferred tax assets before valuation allowance	376,220	383,032
Valuation allowance	(66,265)	(57,659)
Net deferred tax assets	$ 309,955	$ 325,373