Comprehensive Income (Loss) and Changes in Accumulated Other Comprehensive Income (Loss) by Component - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Accumulated other comprehensive income (loss)
Balance	$ 311,299	$ 341,989	$ 346,730
Other comprehensive income (loss)	41,829	(26,570)	(26,125)
Balance	333,772	311,299	341,989
AOCI Attributable to Parent
Accumulated other comprehensive income (loss)
Balance	(114,523)	(87,953)	(61,828)
Other comprehensive income (loss) before reclassifications	31,511	(34,449)
Amounts reclassified from accumulated other comprehensive income (loss), Tax benefit	(6,007)	(4,592)
Other comprehensive income (loss)	41,829	(26,570)	(26,125)
Balance	(72,694)	(114,523)	(87,953)
Accumulated Defined Benefit Plans Adjustment | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss)
Balance	(74,742)	(62,985)
Other comprehensive income (loss) before reclassifications	24,109	(17,855)
Amounts reclassified from accumulated other comprehensive income (loss), Tax benefit	(4,426)	(3,548)
Other comprehensive income (loss)	31,730	(11,757)
Balance	(43,012)	(74,742)	(62,985)
Accumulated Defined Benefit Plans Adjustment | Postretirement Health Care Benefits
Accumulated other comprehensive income (loss)
Balance	(29,585)	(21,773)
Other comprehensive income (loss) before reclassifications	5,197	(9,593)
Amounts reclassified from accumulated other comprehensive income (loss), Tax benefit	(1,581)	(1,044)
Other comprehensive income (loss)	7,894	(7,812)
Balance	(21,691)	(29,585)	(21,773)
Pension and postretirement: Less: amortization of prior service cost
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	808	808
Other comprehensive income (loss)	510	(509)	(510)
Pension and postretirement: Less: amortization of prior service cost | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	808	808
Pension and postretirement: Less: amortization of gain (loss)
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	15,517	11,663
Other comprehensive income (loss)	9,808	(7,370)	(4,528)
Pension and postretirement: Less: amortization of gain (loss) | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	11,239	8,838
Pension and postretirement: Less: amortization of gain (loss) | Postretirement Health Care Benefits
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	4,278	2,825
Accumulated Translation Adjustment
Accumulated other comprehensive income (loss)
Balance	(10,196)	(3,195)
Other comprehensive income (loss) before reclassifications	2,205	(7,001)
Other comprehensive income (loss)	2,205	(7,001)	(4,167)
Balance	$ (7,991)	$ (10,196)	$ (3,195)