Pension and Retirement Plans - Components of net periodic benefit costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Pension
Pension:
Interest cost	$ 70	$ 98	$ 140	$ 195
Expected return on plan assets	(74)	(78)	(148)	(155)
Amortization of:
Amortization of past service costs	2		4
Amortization of net gain (loss)	49	37	98	74
Net periodic benefit cost	47	57	94	114
Post Retirement
Pension:
Service cost	9	9	19	17
Interest cost	11	13	23	26
Amortization of:
Amortization of net gain (loss)	6	(5)	12	(9)
Net periodic benefit cost	26	17	54	34
U.S. | Pension
Pension:
Interest cost	4	6	8	12
Amortization of:
Amortization of net gain (loss)	1	(1)	2	(2)
Net periodic benefit cost	5	5	10	10
U.S. | Post Retirement
Pension:
Service cost	9	9	19	17
Interest cost	11	13	23	26
Amortization of:
Amortization of net gain (loss)	6	(5)	12	(9)
Net periodic benefit cost	26	17	54	34
U.K. | Pension
Pension:
Interest cost	66	92	132	183
Expected return on plan assets	(74)	(78)	(148)	(155)
Amortization of:
Amortization of past service costs	2		4
Amortization of net gain (loss)	48	38	96	76
Net periodic benefit cost	$ 42	$ 52	$ 84	$ 104