Shareholders' Equity - AOCI Changes (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2017 USD ($)
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	$ 4,164,591
Balance at end of period	4,590,350
Unrealized appreciation (depreciation) on securities
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	20,070
Other comprehensive loss before reclassifications	(601)
Net current-period other comprehensive income (loss)	(601)
Balance at end of period	19,469
Defined benefit pension plan
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	(17,770)
Amounts reclassified from accumulated other comprehensive income	340
Net current-period other comprehensive income (loss)	340
Balance at end of period	(17,430)
Accumulated Other Comprehensive Loss
Rollforward of accumulated other comprehensive income (loss), net of tax
Balance at beginning of period	2,300
Other comprehensive loss before reclassifications	(601)
Amounts reclassified from accumulated other comprehensive income	340
Net current-period other comprehensive income (loss)	(261)
Balance at end of period	$ 2,039