Shareholders' Equity - AOCI Changes (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2016	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance		$ 4,895,846
Balance	$ 4,782,100	4,782,100
Unrealized appreciation on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	6,191	17,201
Other comprehensive loss before reclassifications	(1,453)	(12,463)
Net current-period other comprehensive income (loss)	(1,453)	(12,463)
Balance	4,738	4,738
Defined benefit pension plan
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(18,265)	(18,578)
Amounts reclassified from accumulated other comprehensive income	313	626
Net current-period other comprehensive income (loss)	313	626
Balance	(17,952)	(17,952)
Accumulated Other Comprehensive Income.
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(12,074)	(1,377)
Other comprehensive loss before reclassifications	(1,453)	(12,463)
Amounts reclassified from accumulated other comprehensive income	313	626
Net current-period other comprehensive income (loss)	(1,140)	(11,837)
Balance	$ (13,214)	$ (13,214)