Property, Plant, and Equipment, Net (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Property, Plant and Equipment [Line Items]
Land, land improvements, and building improvements held for use	$ 7,604		$ 7,604		$ 0
Property, plant and equipment remaining	13,200		13,200
Depreciation expense	15,754	$ 13,663	30,581	$ 26,118
Second Campus Rockford Illinois [Member]
Property, Plant and Equipment [Line Items]
Construction in progress	40,792		40,792		49,347
Drake Campus Fort Collins Colorado Member
Property, Plant and Equipment [Line Items]
Construction in progress	29,644		29,644		15,584
Office furniture and equipment [Member]
Property, Plant and Equipment [Line Items]
Gross assets acquired on capital leases	1,653		1,653
Accumulated depreciation on capital lease assets	$ 948		$ 948		$ 739