CONSOLIDATED STATEMENTS OF SHAREHOLDERS' 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, 2018	$ 1,996,908	$ 135,343	$ 1,666,609	$ 1,188,971	$ 8,294	$ (1,002,309)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	104,040			104,040
Other comprehensive income (loss)	340				340
Dividends on common stock	(30,775)			(30,775)
Proceeds from stock-based awards	553	24	529
Stock-based compensation expense	2,862	101	2,761
Exercise of stock warrants	0	39	(39)
Treasury stock acquired	(69,648)					(69,648)
Balance, end of period at Mar. 31, 2019	2,004,280	135,507	1,669,860	1,262,236	8,634	(1,071,957)
Balance, beginning of period at Dec. 31, 2018	1,983,089	135,496	1,668,666	1,227,275	2,891	(1,051,239)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	51,098			51,098
Other comprehensive income (loss)	5,743				5,743
Dividends on common stock	(16,137)			(16,137)
Proceeds from stock-based awards	95	8	87
Stock-based compensation expense	1,110	3	1,107
Treasury stock acquired	(20,718)					(20,718)
Balance, end of period at Mar. 31, 2019	2,004,280	135,507	1,669,860	1,262,236	8,634	(1,071,957)
Balance, beginning of period at Sep. 30, 2019	2,032,995	135,540	1,672,417	1,335,909	15,292	(1,126,163)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	103,653			103,653
Other comprehensive income (loss)	(8,831)				(8,831)
Dividends on common stock	(33,374)			(33,374)
Proceeds from stock-based awards	115	6	109
Stock-based compensation expense	3,498	196	3,302
Treasury stock acquired	(112,089)					(112,089)
Balance, end of period at Mar. 31, 2020	1,985,967	135,742	1,675,828	1,406,188	6,461	(1,238,252)
Balance, beginning of period at Dec. 31, 2019	2,050,909	135,720	1,673,666	1,385,179	15,986	(1,159,642)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	37,950			37,950
Other comprehensive income (loss)	(9,525)				(9,525)
Dividends on common stock	(16,941)			(16,941)
Proceeds from stock-based awards	65	3	62
Stock-based compensation expense	2,119	19	2,100
Treasury stock acquired	(78,610)					(78,610)
Balance, end of period at Mar. 31, 2020	$ 1,985,967	$ 135,742	$ 1,675,828	$ 1,406,188	$ 6,461	$ (1,238,252)