Consolidated Statement of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock Par Value	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive (Loss) Income
Beginning balance at Sep. 30, 2018	$ 1,840,551	$ 589	$ 1,318,457	$ 553,014	$ (31,509)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	90,297			90,297
Other comprehensive income (loss), net of tax	23,137				23,137
Comprehensive (loss) income	113,434
Stock-based compensation, net of tax	1,839	1	1,838
Repurchase common stock	(74,659)	(21)	(74,638)
Common stock cash dividends	(28,771)			(28,771)
Ending balance at Mar. 31, 2019	1,852,394	569	1,245,657	614,540	(8,372)
Beginning balance at Dec. 31, 2018	1,812,008	568	1,244,232	584,264	(17,056)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	44,511			44,511
Other comprehensive income (loss), net of tax	8,684				8,684
Comprehensive (loss) income	53,195
Stock-based compensation, net of tax	1,426	1	1,425
Common stock cash dividends	(14,235)			(14,235)
Ending balance at Mar. 31, 2019	1,852,394	569	1,245,657	614,540	(8,372)
Beginning balance at Sep. 30, 2019	1,900,249	563	1,228,714	657,475	13,497
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	(697,344)			(697,344)
Other comprehensive income (loss), net of tax	22,741				22,741
Comprehensive (loss) income	(674,603)
Stock-based compensation, net of tax	1,637	1	1,636
Repurchase common stock	(39,983)	(14)	(39,969)
Common stock cash dividends	(33,654)			(33,654)
Ending balance at Mar. 31, 2020	1,153,464	550	1,190,381	(73,705)	36,238
Beginning balance at Dec. 31, 2019	1,920,669	563	1,229,077	683,682	7,347
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	(740,618)			(740,618)
Other comprehensive income (loss), net of tax	28,891				28,891
Comprehensive (loss) income	(711,727)
Stock-based compensation, net of tax	1,274	1	1,273
Repurchase common stock	(39,983)	(14)	(39,969)
Common stock cash dividends	(16,769)			(16,769)
Ending balance at Mar. 31, 2020	$ 1,153,464	$ 550	$ 1,190,381	$ (73,705)	$ 36,238