Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income tax expense computed at the statutory rate	$ 14,195	$ 18,537
State income taxes, net of federal benefit	2,202	1,580
Tax exempt interest income	(1,371)	(2,038)
Impact of enacted federal income tax rate reduction	13,586	0
Other	29	(2,019)
Income tax expense	$ 28,641	$ 16,060