CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 140,266	$ 114,702	$ 263,965	$ 149,631
Other comprehensive (loss) income, net of tax
Unrealized (loss) gain on available for sale securities, net of tax of $7,366, $(670), $8,217 and $(4,456), respectively	(20,401)	1,408	(22,691)	6,923
Reclassifications of losses to net income on available for sale securities, net of tax of $(3,036), $0, $(858) and $0, respectively	14,801	0	11,647	0
Adjustment to postemployment benefit obligation, net of tax of $(104), $(217), $(240) and $(434), respectively	272	318	512	635
Other comprehensive (loss) income	(5,328)	1,726	(10,532)	7,558
Comprehensive income	$ 134,938	$ 116,428	$ 253,433	$ 157,189