Equity Incentive Plans - Employee Stock Purchase Plan (Details) - Employee Stock - USD ($) $ in Millions					2 Months Ended	6 Months Ended	10 Months Ended	12 Months Ended	257 Months Ended
	Jul. 31, 2017	Jul. 29, 2016	Jan. 29, 2016	Feb. 08, 2012	Sep. 30, 2017	Jul. 29, 2016	Jul. 31, 2017	Sep. 30, 2017	Jul. 31, 2017	Aug. 01, 2017	Sep. 30, 2016	Feb. 07, 2012	Feb. 22, 1996
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized (in shares)				4,000,000						1,250,000	1,000,000	3,000,000	3,000,000
Purchase price of common stock (as a percent)								85.00%
Additional shares of common stock available for purchase (in shares)				1,000,000
Maximum percentage of base pay withheld and applied toward the purchase of shares								10.00%
Shares available for grant (in shares)	0				1,250,000		0	1,250,000	0
Shares expired (in shares)	50,568
Shares issued under employee stock purchase plan (in shares)		117,179	118,548		0	235,727	162,360		3,949,432
Shares issued under employee stock purchase plan, value						$ 1.9	$ 2.0