CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Common Stock	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Balance, beginning of period at Sep. 30, 2015	$ 1,955,679	$ 133,696	$ 1,643,712	$ 829,754	$ 353	$ (651,836)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	76,821			76,821
Other comprehensive income (loss)	(8,939)				(8,939)
Dividends on common stock	(24,735)			(24,735)
Compensation expense related to common stock options	600		600
Proceeds from exercise of common stock options	5,399	250	5,149
Restricted stock expense	2,082	146	1,936
Treasury stock acquired	(44,447)					(44,447)
Balance, end of period at Mar. 31, 2016	1,962,460	134,092	1,651,397	881,840	(8,586)	(696,283)
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	83,316			83,316
Other comprehensive income (loss)	13,435				13,435
Dividends on common stock	(47,995)			(47,995)
Proceeds from exercise of common stock options	6,505	282	6,223
Restricted stock expense	4,171	110	4,061
Exercise of stock warrants	0	124	(124)
Treasury stock acquired	(20,401)					(20,401)
Balance, end of period at Mar. 31, 2017	$ 2,014,762	$ 134,824	$ 1,658,548	$ 979,198	$ 2,279	$ (760,087)