Shareholders' Equity - AOCI Components (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016
Repurchase of shares
Repurchase of common stock (in shares)	0	0
After-tax amounts:
Stockholders' Equity Attributable to Parent	$ 4,475,915	$ 4,560,925
Maximum
Repurchase of shares
Number of common shares authorized to be repurchased	4,000,000
Unrealized appreciation (depreciation) on securities
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	$ 53,293	33,051
After-tax amounts:
Stockholders' Equity Attributable to Parent	33,311	20,899
Defined benefit pension plan
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	(33,537)	(34,112)
After-tax amounts:
Stockholders' Equity Attributable to Parent	(20,737)	(21,103)
Accumulated Other Comprehensive Income (loss)
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	19,756	(1,061)
After-tax amounts:
Stockholders' Equity Attributable to Parent	$ 12,574	$ (204)