Senior Notes and Other Long-Term Debt - Components of Long-Term Debt (Detail) - USD ($)	Mar. 31, 2018	Mar. 31, 2017
Debt Instrument [Line Items]
Total debt	$ 1,062,401,000	$ 879,422,000
Unamortized premium/(discount and debt issuance costs), net	(38,696,000)	(30,651,000)
Less: current portion of long-term debt	45,300,000	288,000
Total long-term debt	978,405,000	848,483,000
2025 Notes [Member]
Debt Instrument [Line Items]
Total debt	700,000,000
2020 Notes [Member]
Debt Instrument [Line Items]
Total debt		575,000,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Total debt	0	0
Ex-Im Credit Facility [Member]
Debt Instrument [Line Items]
Total debt	$ 362,401,000	304,134,000
Other [Member]
Debt Instrument [Line Items]
Total debt		$ 288,000