Impact on Previously Issued Financial Statements for the Correction of an Error, Revised Consolidated Balance Sheet (Details) - USD ($)	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
ASSETS [Abstract]
Contract assets	$ 24,272,000		$ 23,206,000
Income tax receivable	11,572,000		7,972,000
Prepaid expenses and other current assets	10,200,000		8,608,000
Long-term core inventory - net			0
Long-term deferred income taxes	7,345,000		6,698,000
Long-term contract assets	230,438,000		222,731,000
Other assets	866,000		804,000
TOTAL ASSETS	584,013,000		552,427,000
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Accrued liabilities	10,622,000		12,048,000
TOTAL LIABILITIES	301,849,000		265,547,000
Retained earnings	77,274,000		78,510,000
TOTAL SHAREHOLDERS' EQUITY	282,164,000		286,880,000
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 584,013,000		552,427,000
Historical Misapplication of GAAP Related to Timing of Recognizing Certain Expenses Incurred in Connection with New Business [Member]
ASSETS [Abstract]
Contract assets		$ 24,316,000
Income tax receivable		9,761,000	7,972,000
Prepaid expenses and other current assets		10,255,000	15,104,000	$ 13,088,000
Long-term core inventory - net			298,294,000	258,421,000
Long-term deferred income taxes		6,724,000	6,937,000	8,367,000
Long-term contract assets		222,462,000
Other assets		810,000	21,995,000	22,381,000
TOTAL ASSETS		559,934,000	505,908,000	445,090,000
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Accrued liabilities			12,048,000	10,376,000
TOTAL LIABILITIES			219,770,000	187,757,000
Retained earnings		73,761,000	77,768,000	58,942,000	$ 19,274,000	$ 1,262,000
TOTAL SHAREHOLDERS' EQUITY		281,419,000	286,138,000	257,333,000
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY		559,934,000	505,908,000	445,090,000
Historical Misapplication of GAAP Related to Timing of Recognizing Certain Expenses Incurred in Connection with New Business [Member] | As Previously Reported [Member]
ASSETS [Abstract]
Contract assets		16,542,000
Income tax receivable		9,416,000	7,796,000
Prepaid expenses and other current assets		13,148,000	11,491,000	9,848,000
Long-term core inventory - net			301,656,000	262,922,000
Long-term deferred income taxes		10,343,000	10,556,000	13,546,000
Long-term contract assets		207,792,000
Other assets		6,406,000	7,392,000	6,990,000
TOTAL ASSETS		549,253,000	494,497,000	436,139,000
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Accrued liabilities			11,799,000	10,077,000
TOTAL LIABILITIES			219,521,000	187,458,000
Retained earnings		63,080,000	66,606,000	50,290,000	11,825,000	1,262,000
TOTAL SHAREHOLDERS' EQUITY		270,738,000	274,976,000	248,681,000
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY		549,253,000	494,497,000	436,139,000
Historical Misapplication of GAAP Related to Timing of Recognizing Certain Expenses Incurred in Connection with New Business [Member] | Adjustment [Member]
ASSETS [Abstract]
Contract assets		7,774,000
Income tax receivable		345,000	176,000
Prepaid expenses and other current assets		(2,893,000)	3,613,000	3,240,000
Long-term core inventory - net			(3,362,000)	(4,501,000)
Long-term deferred income taxes		(3,619,000)	(3,619,000)	(5,179,000)
Long-term contract assets		14,670,000
Other assets		(5,596,000)	14,603,000	15,391,000
TOTAL ASSETS		10,681,000	11,411,000	8,951,000
LIABILITIES AND SHAREHOLDERS' EQUITY [Abstract]
Accrued liabilities			249,000	299,000
TOTAL LIABILITIES			249,000	299,000
Retained earnings		10,681,000	11,162,000	8,652,000	$ 7,449,000	$ 0
TOTAL SHAREHOLDERS' EQUITY		10,681,000	11,162,000	8,652,000
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY		$ 10,681,000	$ 11,411,000	$ 8,951,000