Shareholders' Equity - AOCI Changes (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Jun. 30, 2017	Jun. 30, 2017
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance		$ 4,560,925
Balance	$ 4,249,904	4,249,904
Unrealized appreciation (depreciation) on securities
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	22,804	20,899
Other comprehensive loss before reclassifications	(6,899)	(4,994)
Net current-period other comprehensive income (loss)	(6,899)	(4,994)
Balance	15,905	15,905
Defined benefit pension plan
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	(20,371)	(21,103)
Amounts reclassified from accumulated other comprehensive income	365	1,097
Net current-period other comprehensive income (loss)	365	1,097
Balance	(20,006)	(20,006)
Accumulated Other Comprehensive Loss
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	2,433	(204)
Other comprehensive loss before reclassifications	(6,899)	(4,994)
Amounts reclassified from accumulated other comprehensive income	365	1,097
Net current-period other comprehensive income (loss)	(6,534)	(3,897)
Balance	$ (4,101)	$ (4,101)