Employee Benefit Plans - Retired Officers' Bonus Plan and Retired VP Bonus Plan (Details) - Other Postretirement Benefits Plan - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Officer
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, funded (unfunded) status of plan	$ 3,800	$ 3,700
Retired Officers' Bonus Plan | Officer
Defined Benefit Plan Disclosure [Line Items]
Total pension expense	600	700
Benefits paid	600	800	$ 1,500
Retired Vice Presidents' Bonus Plan
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	1,000	900	900
Retired Vice Presidents' Bonus Plan | Officer
Defined Benefit Plan Disclosure [Line Items]
Total pension expense	200	200	200
Benefits paid	100
Officer Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Total pension expense	9,814	13,190	11,743
Benefits paid	3,785	3,742	3,419
Defined benefit plan, funded (unfunded) status of plan	(119,600)	(120,300)
Benefit obligation	119,609	120,341	126,886	$ 118,089
Fair value of plan assets	0	0	$ 0	$ 0
Retired Officers' Bonus Plan and Retired Vice-President Bonus Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, funded (unfunded) status of plan	(4,800)	(4,600)
Fair value of plan assets	$ 0	$ 0