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, 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	100,941			100,941
Other comprehensive income (loss)	3,884
Other comprehensive income (loss)	2,112				2,112
Dividends on common stock ($0.32 per share)	(26,855)			(26,855)
Proceeds from stock-based awards	1,232	56	1,176
Stock-based compensation expense	2,534	211	2,323
Exercise of stock warrants	0	109	(109)
Treasury stock acquired	(96,979)					(96,979)
Balance, end of period at Mar. 31, 2018	1,988,673	135,334	1,664,275	1,115,204	8,899	(935,039)
Balance, beginning of period at Dec. 31, 2017	2,009,618	135,275	1,661,866	1,081,517	8,004	(877,044)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	49,271			49,271
Other comprehensive income (loss)	895
Other comprehensive income (loss)	(877)				(877)
Dividends on common stock ($0.32 per share)	(13,812)			(13,812)
Proceeds from stock-based awards	946	42	904
Stock-based compensation expense	1,522	17	1,505
Treasury stock acquired	(57,995)					(57,995)
Balance, end of period at Mar. 31, 2018	1,988,673	135,334	1,664,275	1,115,204	8,899	(935,039)
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 ($0.32 per share)	(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 ($0.32 per share)	(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)