Other Comprehensive Income (Loss) - Reclassification out of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
(Loss) gain on securities transactions, net	$ 116	$ 0	$ 55	$ 411
Compensation and employee benefits expense	(16,525)	(15,980)	(32,146)	(30,989)
Total before tax	108	7	37	426
Income tax benefit	(122)	(2)	(214)	(148)
Net of tax	(14)	5	(177)	278
Accumulated other Comprehensive (Loss) Income Components | Reclassification adjustment for gains included in net income
Accumulated Other Comprehensive Income (Loss) [Line Items]
(Loss) gain on securities transactions, net	116	0	55	411
Accumulated other Comprehensive (Loss) Income Components | Reclassification adjustment of actuarial net (loss) gain included in net income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Compensation and employee benefits expense	$ 8	$ (7)	$ 18	$ (15)