Consolidated Statements of Changes in Stockholder's Equity (Unaudited) - USD ($) $ in Thousands	Total	Retained Earnings	Accumulated other comprehensive loss, net of tax
Balance at beginning of year at Sep. 30, 2016	$ 439,664	$ 491,022	$ (51,358)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,286	20,286	0
Other comprehensive loss	(14,552)	0	(14,552)
Balance at end of year at Mar. 31, 2017	445,398	511,308	(65,910)
Balance at beginning of year at Dec. 31, 2016			(66,162)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	10,294
Other comprehensive loss	252
Balance at end of year at Mar. 31, 2017	445,398	511,308	(65,910)
Balance at beginning of year at Sep. 30, 2017	475,914	522,094	(46,180)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	15,462	15,462	0
Other comprehensive loss	(29,180)
Other comprehensive income (loss) including impact of tax act	(17,472)	0	(17,472)
Balance at end of year at Mar. 31, 2018	473,904	549,264	(75,360)
Balance at beginning of year at Dec. 31, 2017			(65,411)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	11,784
Other comprehensive loss	(9,949)
Balance at end of year at Mar. 31, 2018	473,904	549,264	(75,360)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of tax effects resulting from the adoption of ASU No. 2018-02 | Accounting Standards Update 2018-02	$ 0	$ 11,708	$ (11,708)