Employee Benefit Plans - Components of Net Periodic Benefit Cost and Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Mar. 31, 2014
Pension Plan [Member]
Components of net periodic benefit cost
Service cost	$ 97	$ 97	$ 194	$ 194
Interest cost	1,546	1,415	3,092	2,830
Expected return on plan assets	(1,710)	(1,823)	(3,420)	(3,646)
Amortization of net loss(1)	370	303	740	606
Net periodic benefit cost (credit)	$ 303	$ (8)	606	(16)
Weighted average assumptions used to determine benefit obligations
Discount rate	4.40%	4.15%
Expected long-term return on plan assets	7.50%	7.50%
Other Postretirement Benefit Plan, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost	$ 347	$ 299	694	598
Interest cost	299	235	598	470
Amortization of net loss(1)	(49)	(125)	(98)	(250)
Amortization of prior service cost (credit)	(439)	(439)	(878)	(878)
Net periodic benefit cost (credit)	$ 158	$ (30)	$ 316	$ (60)
Retiree Medical Plan
Weighted average assumptions used to determine benefit obligations
Discount rate			4.69%	4.29%
Death Benefit Plan
Weighted average assumptions used to determine benefit obligations
Discount rate			4.74%	4.48%