Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income		$ 44,511	$ 40,532	$ 90,297	$ 69,762
Securities available for sale:
Net unrealized holding gain (loss) arising during the period		11,523	(12,425)	30,192	(21,070)
Reclassification adjustment for net loss realized in net income		0	8	513	9
Income tax (expense) benefit		(2,839)	3,062	(7,568)	6,345
Net change in unrealized gain (loss) on securities available for sale		8,684	(9,355)	23,137	(14,716)
Net unrealized holding gain arising during the period	[1]	0	0	0	145
Income tax expense	[1]	0	0	0	(55)
Net change in defined benefit pension plan obligation	[1]	0	0	0	90
Other comprehensive income (loss), net of tax		8,684	(9,355)	23,137	(14,626)
Comprehensive income		$ 53,195	$ 31,177	$ 113,434	$ 55,136

[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.