Postretirement Benefits - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax [Abstract]
Net actuarial gains (losses)	$ 400	$ (300)
Cumulative unrecognized investment gains (losses)		(900)
Settlement loss	259
Curtailment gain		$ 852
Maximum
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax [Abstract]
Cumulative unrecognized investment gains (losses)	$ (100)