Shareholders' Equity - AOCI Changes (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	$ 4,895,846
Balance	4,830,293	$ 4,895,846
Unrealized appreciation on securities
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	9,066	27,021
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	17,201
Other comprehensive loss before reclassifications	(11,010)
Net current-period other comprehensive loss	(11,010)
Balance	6,191	17,201
Defined Benefit Pension Plan
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	(29,651)	(30,144)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(18,578)
Amounts reclassified from accumulated other comprehensive income (loss)	313
Net current-period other comprehensive loss	313
Balance	(18,265)	(18,578)
Accumulated Other Comprehensive Income.
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	(20,585)	(3,123)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(1,377)
Other comprehensive loss before reclassifications	(11,010)
Amounts reclassified from accumulated other comprehensive income (loss)	313
Net current-period other comprehensive loss	(10,697)
Balance	$ (12,074)	$ (1,377)