Basis of Financial Statement Presentation - ASC 606 Adjustment Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 349,233	$ 301,568	$ 676,849	$ 575,514
Operating costs and expenses:
Cost of sales	235,668	216,664	468,463	416,493
Research and development	10,995	9,536	21,683	18,783
Operating Income (Loss)	50,000	31,597	85,176	59,204
Income tax expense	2,000	2,864	6,600	4,004
Net income (loss)	$ 37,141	$ 12,819	$ 72,361	$ 233,258
Net income (loss) per basic share (in usd per share)	$ 0.64	$ 0.26	$ 1.26	$ 4.80
Earnings Per Share, Diluted	$ 0.63	$ 0.22	$ 1.22	$ 4.01
As Previously Reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		$ 301,471		$ 575,471
Operating costs and expenses:
Cost of sales		216,395		415,958
Research and development		9,662		19,052
Operating Income (Loss)		31,643		59,427
Income tax expense		2,880		4,030
Net income (loss)		12,849		$ 233,455
Net income (loss) per basic share (in usd per share)				$ 4.02
ASC 606 Adjustments | ASC 606 Adjustments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		97		$ 43
Operating costs and expenses:
Cost of sales		269		535
Research and development		(126)		(269)
Operating Income (Loss)		(46)		(223)
Income tax expense		(16)		(26)
Net income (loss)		$ (30)		$ (197)
Net income (loss) per basic share (in usd per share)				$ (0.01)