Shareholders' Equity - AOCI Components (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Repurchase of shares
Repurchase of common stock (in shares)	0	0
After-tax amounts:
Accumulated other comprehensive income	$ 8,942		$ 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	$ 39,327		31,700
After-tax amounts:
Accumulated other comprehensive income	25,727		20,070
Defined benefit pension plan
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	(27,491)		(28,873)
After-tax amounts:
Accumulated other comprehensive income	(16,785)		(17,770)
Accumulated Other Comprehensive Loss
Pre-tax amounts:
Accumulated other comprehensive income (loss) before tax	11,836		2,827
After-tax amounts:
Accumulated other comprehensive income	$ 8,942		$ 2,300