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	9,993	9,993	0
Other comprehensive loss	(14,804)	0	(14,804)
Balance at end of year at Dec. 31, 2016	434,853	501,015	(66,162)
Balance at beginning of year at Sep. 30, 2017	475,914	522,094	(46,180)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	3,678	3,678	0
Other comprehensive loss	(19,230)
Other comprehensive income (loss) including impact of tax act	(7,522)	0	(7,522)
Balance at end of year at Dec. 31, 2017	472,070	537,480	(65,410)
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)