Employee Benefit Plans (Narratives) (Details)	12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)
Compensation of related costs, Share-based Payments [Abstract]
Length of service requirement	1 year	1 year	1 year
Employee stock ownership plan (ESOP), compensation expense	$ 11,400,000	$ 10,700,000	$ 11,600,000
Employee stock ownership plan (ESOP), cash contributions to ESOP	1,000,000	200,000	400,000
Cash contributions to ESOP non leveraged	11,000,000	$ 11,000,000	$ 4,000,000
Unallocated common stock dividend, debt repayment	$ 100,000
Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]
Defined benefit plan, health care cost trend rate assumed for next fiscal year	6.90%	4.50%
Defined benefit plan, ultimate health care cost trend rate	7.10%	7.30%
Defined benefit plan, year rate reaches ultimate trend rate	2038	2029
Defined benefit plan, effect of one percentage point increase on accumulated postretirement benefit obligation	$ 287,000
Defined benefit plan, effect of one percentage point increase on service and interest cost components	25,000
Defined benefit plan, effect of one percentage point decrease on accumulated postretirement benefit obligation	323,000
Defined benefit plan, effect of one percentage point decrease on service and interest cost components	$ 29,000
Post retirement health insurance [Member]
Deferred Compensation Arrangements [Abstract]
Age requirement for retiring	65
Deferred compensation arrangement with individual, requisite service period	20 years
Lifetime maximum benefit paid per individual	$ 20,000
Post retirement life insurance [Member]
Deferred Compensation Arrangements [Abstract]
Age requirement for retiring	65
Deferred compensation arrangement with individual, requisite service period	20 years
Lifetime maximum benefit paid per individual	$ 2,000
Additional benefit stipulation for life insurance	10 years
Additional benefit requirement amount	$ 100