Business Segments, Significant Customer and Geographic Information - Percentage of Revenue by Geographic region (Details) - Total revenues	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Percentage of revenues by geographic region derived from shipments to, or contract, service and other revenues from, external customers located outside the U.S.
Percentage of total net sales and contract revenues	1.00%	2.00%
Canada
Percentage of revenues by geographic region derived from shipments to, or contract, service and other revenues from, external customers located outside the U.S.
Percentage of total net sales and contract revenues		1.00%
Europe
Percentage of revenues by geographic region derived from shipments to, or contract, service and other revenues from, external customers located outside the U.S.
Percentage of total net sales and contract revenues	1.00%	1.00%