Senior Notes and Other Long-Term Debt - Components of Long-Term Debt (Detail) - USD ($)	Dec. 31, 2017	Sep. 21, 2017	Mar. 31, 2017
Debt Instrument [Line Items]
Other	$ 299,000		$ 288,000
Total debt	1,062,700,000		879,422,000
Unamortized premium/(discount and debt issuance costs), net	(40,963,000)		(30,651,000)
Less: current portion of long-term debt	45,599,000		288,000
Total long-term debt	976,138,000		848,483,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit Facility	0		0
Ex-Im Credit Facility [Member]
Debt Instrument [Line Items]
Credit Facility	362,401,000		304,134,000
2025 Notes [Member]
Debt Instrument [Line Items]
Notes	$ 700,000,000	$ 700,000,000
2020 Notes [Member]
Debt Instrument [Line Items]
Notes		$ 575,000,000	$ 575,000,000

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

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Amount of unamortized debt discount (premium) and debt issuance costs.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt classified as other.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, including financing fees.

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