Consolidated Statement of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Comprehensive Income	Common Stock Par Value	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Sep. 30, 2016	$ 1,663,391		$ 587	$ 1,312,347	$ 344,923	$ 5,534
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	36,903	$ 36,903			36,903
Other comprehensive (loss), net of tax	(13,310)	(13,310)				(13,310)
Comprehensive income	23,593	23,593
Stock-based compensation, net of tax	1,635			1,635
Common stock cash dividends	(9,981)				(9,981)
Ending balance at Dec. 31, 2016	1,678,638		587	1,313,982	371,845	(7,776)
Beginning balance at Sep. 30, 2017	1,755,000		588	1,314,039	445,747	(5,374)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	29,230	29,230			29,230
Other comprehensive (loss), net of tax	(5,271)	(5,271)				(5,271)
Comprehensive income	23,959	$ 23,959
Stock-based compensation, net of tax	684			684
Common stock cash dividends	(11,770)				(11,770)
Ending balance at Dec. 31, 2017	$ 1,767,873		$ 588	$ 1,314,723	$ 463,207	$ (10,645)