Shareholders' Equity - AOCI Components (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Repurchase of shares
Repurchase of common stock (in shares)	0	0
After-tax amounts:
Accumulated other comprehensive income (loss), after tax	$ (5,221)		$ 2,300
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	$ 19,908		31,700
After-tax amounts:
Accumulated other comprehensive income (loss), after tax	11,901		20,070
Defined benefit pension plan
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	(27,952)		(28,873)
After-tax amounts:
Accumulated other comprehensive income (loss), after tax	(17,122)		(17,770)
Accumulated Other Comprehensive Loss
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	(8,044)		2,827
After-tax amounts:
Accumulated other comprehensive income (loss), after tax	$ (5,221)		$ 2,300