Net Periodic Benefit Cost (Components of net periodic pension cost and net periodic postretirement benefit cost) (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Other	$ (4,082)	$ 0
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Asset categories:	326,745	321,440
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	2,580	1,779	$ 2,187
Interest cost	16,488	16,648	13,926
Expected return on plan assets	(21,483)	(22,428)	(19,783)
Net amortization	3,083	3,190	10
Net periodic pension cost (benefit)	690	(621)	(1,208)
Defined Benefit Plan, Plan with Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligation		471,871
Fair value of plan assets		321,440
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]
Accumulated benefit obligation	457,323	463,412
Asset categories:	$ 326,745	321,440	$ 317,868
Fair value of plan assets		$ 321,440
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	3.49%	3.65%	4.30%
Expected long-term rate of return on plan assets	6.77%	7.23%	7.22%
Rate of compensation increase	0.39%	0.39%	0.44%
Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 126	$ 152	$ 189
Net amortization	(5)	0	89
Net periodic pension cost (benefit)	$ 121	$ 152	$ 278
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	3.75%	3.60%