Other Comprehensive Income (Loss) - Reclassification out of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
(Loss) gain on securities transactions, net	$ (60)	$ 411
Compensation and employee benefits expense	(15,621)	(15,010)
Total before tax	(69)	418
Income tax benefit	(81)	(149)
Net of tax	(150)	269
Accumulated other Comprehensive (Loss) Income Components | Reclassification adjustment for (loss) gain included in net income
Accumulated Other Comprehensive Income (Loss) [Line Items]
(Loss) gain on securities transactions, net	(60)	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	$ (9)	$ 7