Senior Notes and Other Long-Term Debt - Components of Long-Term Debt (Detail) - USD ($)	Sep. 30, 2018	Mar. 31, 2018
Debt Instrument [Line Items]
Total debt	$ 1,231,015,000	$ 1,062,401,000
Unamortized discount and debt issuance costs	(31,715,000)	(38,696,000)
Less: current portion of long-term debt	47,702,000	45,300,000
Total long-term debt	1,151,598,000	978,405,000
2025 Notes [Member]
Debt Instrument [Line Items]
Total debt	700,000,000	700,000,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Total debt	225,000,000	0
Ex-Im Credit Facility [Member]
Debt Instrument [Line Items]
Total debt	$ 306,015,000	$ 362,401,000