Shareholders' Equity - AOCI Changes (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period		$ 4,164,591
Balance at end of period	$ 4,442,491	4,442,491
Unrealized appreciation (depreciation) on securities
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	11,901	20,070
Other comprehensive income before reclassifications	13,826	5,657
Net current-period other comprehensive income	13,826	5,657
Balance at end of period	25,727	25,727
Defined benefit pension plan
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	(17,122)	(17,770)
Amounts reclassified from accumulated other comprehensive income	337	985
Net current-period other comprehensive income	337	985
Balance at end of period	(16,785)	(16,785)
Accumulated Other Comprehensive Loss
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	(5,221)	2,300
Other comprehensive income before reclassifications	13,826	5,657
Amounts reclassified from accumulated other comprehensive income	337	985
Net current-period other comprehensive income	14,163	6,642
Balance at end of period	$ 8,942	$ 8,942