Commitments and Contingencies - Contingencies (Details) - Employee Stock - USD ($) $ / shares in Units, $ in Millions			2 Months Ended	6 Months Ended	10 Months Ended	12 Months Ended	257 Months Ended
	Jul. 29, 2016	Jan. 29, 2016	Sep. 30, 2017	Jul. 29, 2016	Jul. 31, 2017	Sep. 30, 2016	Jul. 31, 2017	Aug. 01, 2017	Feb. 08, 2012	Feb. 07, 2012	Feb. 22, 1996
Loss Contingency [Abstract]
Shares issued under employee stock purchase plan (in shares)	117,179	118,548	0	235,727	162,360		3,949,432
Share price (in dollars per share)	$ 8.02	$ 8.00		$ 8.02
Shares issued under employee stock purchase plan, value				$ 1.9	$ 2.0
Commissions or other fees paid				$ 0.0
Shares authorized (in shares)						1,000,000		1,250,000	4,000,000	3,000,000	3,000,000
Employee Stock Purchase Plan, Rescission Rights
Loss Contingency [Abstract]
Rescission rights, shares purchased, period						12 months