CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-in Capital	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	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	157,894				157,894
Other comprehensive income (loss)	3,843						3,843
Dividends on common stock	(46,749)				(46,749)
Proceeds from stock-based awards	712		37	675
Stock-based compensation expense	4,061		108	3,953
Exercise of stock warrants	0		39	(39)
Treasury stock acquired	(103,935)							(103,935)
Balance, end of period at Jun. 30, 2019	2,012,734		135,527	1,671,198	1,300,116		12,137	(1,106,244)
Balance, beginning of period at Mar. 31, 2019	2,004,280		135,507	1,669,860	1,262,236		8,634	(1,071,957)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	53,854				53,854
Other comprehensive income (loss)	3,503						3,503
Dividends on common stock	(15,974)				(15,974)
Proceeds from stock-based awards	159		13	146
Stock-based compensation expense	1,199		7	1,192
Treasury stock acquired	(34,287)							(34,287)
Balance, end of period at Jun. 30, 2019	2,012,734		135,527	1,671,198	1,300,116		12,137	(1,106,244)
Balance, beginning of period at Sep. 30, 2019	2,032,995	$ (21,945)	135,540	1,672,417	1,335,909	$ (21,945)	15,292	(1,126,163)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	139,095				139,095
Other comprehensive income (loss)	(2,732)						(2,732)
Dividends on common stock	(49,935)				(49,935)
Proceeds from stock-based awards	141		8	133
Stock-based compensation expense	5,019		196	4,823
Treasury stock acquired	(112,129)							(112,129)
Balance, end of period at Jun. 30, 2020	1,990,509		135,744	1,677,373	1,403,124		12,560	(1,238,292)
Balance, beginning of period at Mar. 31, 2020	1,964,612		135,742	1,675,828	1,384,833		6,461	(1,238,252)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	34,852				34,852
Other comprehensive income (loss)	6,099						6,099
Dividends on common stock	(16,561)				(16,561)
Proceeds from stock-based awards	26		2	24
Stock-based compensation expense	1,521			1,521
Treasury stock acquired	(40)							(40)
Balance, end of period at Jun. 30, 2020	$ 1,990,509		$ 135,744	$ 1,677,373	$ 1,403,124		$ 12,560	$ (1,238,292)