Retirement Benefit Plans - Asset Allocation of Defined Benefit Pension Plan Assets and Fair Values and Levels in Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 21,691	$ 21,344	$ 20,896
Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	19,516	19,159
Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	2,175	2,185	$ 2,045
Large value
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	681	492
Large value | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	681	492
Large value | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Large blend
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	9,788	9,593
Large blend | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	9,788	9,593
Large blend | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Large growth
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	470	503
Large growth | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	470	503
Large growth | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Mid blend
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	79	57
Mid blend | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	79	57
Mid blend | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Small blend
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	111	56
Small blend | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	111	56
Small blend | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Foreign large blend
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,731	1,565
Foreign large blend | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,731	1,565
Foreign large blend | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Diversified emerging markets
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	19	18
Diversified emerging markets | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	19	18
Diversified emerging markets | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Inflation protected bond
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,089	537
Inflation protected bond | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,089	537
Inflation protected bond | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Intermediate term bond
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	7,240	7,747
Intermediate term bond | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	5,065	5,562
Intermediate term bond | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	2,175	2,185
High inflation bond
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	187	360
High inflation bond | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	187	360
High inflation bond | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Emerging markets bonds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	77	66
Emerging markets bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	77	66
Emerging markets bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	0
Short-term bonds
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	219	350
Short-term bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	219	350
Short-term bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 0	$ 0