Shareholders' Equity - AOCI Changes (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Jun. 30, 2016	Jun. 30, 2016
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance		$ 4,895,846
Balance	$ 4,701,462	4,701,462
Unrealized appreciation (depreciation) on securities
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	4,738	17,201
Other comprehensive income (loss) before reclassifications	9,744	(2,719)
Net current-period other comprehensive income (loss)	9,744	(2,719)
Balance	14,482	14,482
Defined benefit pension plan
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	(17,952)	(18,578)
Amounts reclassified from accumulated other comprehensive income	314	940
Net current-period other comprehensive income (loss)	314	940
Balance	(17,638)	(17,638)
Accumulated other comprehensive income (loss)
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	(13,214)	(1,377)
Other comprehensive income (loss) before reclassifications	9,744	(2,719)
Amounts reclassified from accumulated other comprehensive income	314	940
Net current-period other comprehensive income (loss)	10,058	(1,779)
Balance	$ (3,156)	$ (3,156)