CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Balance, beginning of period at Sep. 30, 2016	$ 1,975,731	$ 134,308	$ 1,648,388	$ 943,877	$ (11,156)	$ (739,686)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	127,428			127,428
Other comprehensive income (loss)	13,634				13,634
Dividends on common stock	(61,341)			(61,341)
Proceeds from exercise of common stock options	7,078	309	6,769
Restricted stock expense	5,126	105	5,021
Exercise of stock warrants	0	225	(225)
Treasury stock acquired	(46,470)					(46,470)
Balance, end of period at Jun. 30, 2017	2,021,186	134,947	1,659,953	1,009,964	2,478	(786,156)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment pursuant to adoption of ASU 2018-02	0			(1,772)	1,772
Balance, beginning of period at Sep. 30, 2017	2,005,688	134,958	1,660,885	1,042,890	5,015	(838,060)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	152,335			152,335
Other comprehensive income (loss)	1,350				1,350
Dividends on common stock	(41,091)			(41,091)
Proceeds from exercise of common stock options	1,288	60	1,228
Restricted stock expense	3,634	215	3,419
Exercise of stock warrants	0	111	(111)
Treasury stock acquired	(136,941)					(136,941)
Balance, end of period at Jun. 30, 2018	$ 1,986,263	$ 135,344	$ 1,665,421	$ 1,152,362	$ 8,137	$ (975,001)