Shareholders' Equity - AOCI Changes (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	$ 4,560,925
Balance	4,475,915
Unrealized appreciation (depreciation) on securities
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	20,899
Other comprehensive income before reclassifications	12,412
Net current-period other comprehensive Income	12,412
Balance	33,311
Defined benefit pension plan
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	(21,103)
Amounts reclassified from accumulated other comprehensive income (loss)	366	$ 313
Net current-period other comprehensive Income	366
Balance	(20,737)
Accumulated Other Comprehensive Income (loss)
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance	(204)
Other comprehensive income before reclassifications	12,412
Amounts reclassified from accumulated other comprehensive income (loss)	366
Net current-period other comprehensive Income	12,778
Balance	$ 12,574