Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Mar. 31, 2018	Mar. 31, 2017
Current Assets -
Cash and Cash Equivalents	$ 10,002	$ 9,315
Restricted Cash		38,753
Accounts and Notes Receivable	174,550	141,685
Inventories	238,869	258,159
Income Tax Receivable	5,924	5,750
Prepaid and Other Current Assets	7,751	5,073
Total Current Assets	437,096	458,735
Property, Plant, and Equipment -	2,636,249	2,586,528
Less: Accumulated Depreciation	(1,011,511)	(991,229)
Property, Plant, and Equipment, net	1,624,738	1,595,299
Notes Receivable	3,144	115
Investment in Joint Venture	60,482	60,558
Goodwill and Intangible Assets, net	237,738	239,342
Other Assets	16,314	13,954
Total Assets	2,379,512	2,368,003
Current Liabilities -
Accounts Payable	92,479	73,459
Accrued Liabilities	62,223	105,870
Total Current Liabilities	154,702	179,329
Long-term Debt	631,257	620,922
Other Long-term Liabilities	31,099	31,096
Deferred Income Taxes	129,851	118,966
Total Liabilities	946,909	950,313
Total Stockholders’ Equity	1,432,603	1,417,690	$ 1,203,450
Liabilities and Stockholders' Equity, Total	2,379,512	2,368,003
Eliminations [Member]
Current Assets -
Income Tax Receivable		(103,760)
Total Current Assets		(103,760)
Investments in Subsidiaries and Receivables from Affiliates	(2,697,818)	(3,481,149)
Total Assets	(2,697,818)	(3,584,909)
Current Liabilities -
Income Tax Payable		(103,760)
Total Current Liabilities		(103,760)
Payables to Affiliates	(6,060,966)	(6,370,576)
Total Liabilities	(6,060,966)	(6,474,336)
Total Stockholders’ Equity	3,363,148	2,889,427
Liabilities and Stockholders' Equity, Total	(2,697,818)	(3,584,909)
Parent [Member]
Current Assets -
Cash and Cash Equivalents	7,584	5,784
Restricted Cash		38,753
Accounts and Notes Receivable	394	407
Income Tax Receivable	5,924	109,510
Prepaid and Other Current Assets	379	665
Total Current Assets	14,281	155,119
Property, Plant, and Equipment -	4,521	3,188
Less: Accumulated Depreciation	(1,167)	(1,089)
Property, Plant, and Equipment, net	3,354	2,099
Investment in Joint Venture	70	70
Investments in Subsidiaries and Receivables from Affiliates	2,385,185	2,718,809
Other Assets	5,319	5,417
Total Assets	2,408,209	2,881,514
Current Liabilities -
Accounts Payable	6,586	5,591
Accrued Liabilities	19,211	67,387
Total Current Liabilities	25,797	72,978
Long-term Debt	631,257	620,922
Other Long-term Liabilities	47	124
Payables to Affiliates	312,633	762,340
Deferred Income Taxes	5,872	7,460
Total Liabilities	975,606	1,463,824
Total Stockholders’ Equity	1,432,603	1,417,690
Liabilities and Stockholders' Equity, Total	2,408,209	2,881,514
Guarantor Subsidiaries [Member]
Current Assets -
Cash and Cash Equivalents	2,418	3,531
Accounts and Notes Receivable	174,156	141,278
Inventories	238,869	258,159
Prepaid and Other Current Assets	7,372	4,408
Total Current Assets	422,815	407,376
Property, Plant, and Equipment -	2,631,728	2,583,340
Less: Accumulated Depreciation	(1,010,344)	(990,140)
Property, Plant, and Equipment, net	1,621,384	1,593,200
Notes Receivable	3,144	115
Investment in Joint Venture	60,412	60,488
Investments in Subsidiaries and Receivables from Affiliates	312,633	762,340
Goodwill and Intangible Assets, net	237,738	239,342
Other Assets	10,995	8,537
Total Assets	2,669,121	3,071,398
Current Liabilities -
Accounts Payable	85,893	67,868
Accrued Liabilities	43,012	38,483
Income Tax Payable		103,760
Total Current Liabilities	128,905	210,111
Other Long-term Liabilities	31,052	30,972
Payables to Affiliates	5,748,333	5,608,236
Deferred Income Taxes	123,979	111,506
Total Liabilities	6,032,269	5,960,825
Total Stockholders’ Equity	(3,363,148)	(2,889,427)
Liabilities and Stockholders' Equity, Total	$ 2,669,121	$ 3,071,398