Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net (loss) income		$ (680,808)	$ 167,365	$ 157,916
Securities available for sale:
Net unrealized holding gain (loss) arising during the period		37,630	59,550	(32,816)
Reclassification adjustment for net (gain) loss realized in net income		(7,890)	178	(6)
Income tax (expense) benefit		(7,332)	(14,722)	9,244
Net change in unrealized gain (loss) on securities available for sale		22,408	45,006	(23,578)
Defined benefit pension plan obligation ¹:
Net unrealized holding loss arising during the period	[1]	0	0	(329)
Income tax benefit	[1]	0	0	125
Net change in unrealized loss in defined benefit pension plan obligation	[1]	0	0	(204)
Other comprehensive income (loss), net of tax	[1]	22,408	45,006	(23,782)
Comprehensive (loss) income		$ (658,400)	$ 212,371	$ 134,134

[1] The Company's Board of Directors voted to terminate the defined benefit pension plan ("Pension Plan") effective February 1, 2018. Transfer of all Pension Plan assets, liabilities and administrative responsibilities were completed as of September 30, 2018.