Leases - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Leased Assets [Line Items]
Lease expenses for operating lease	$ 59,590us-gaap_OperatingLeasesRentExpenseNet	$ 45,093us-gaap_OperatingLeasesRentExpenseNet	$ 44,177us-gaap_OperatingLeasesRentExpenseNet
Total royalties	50,535us-gaap_RoyaltyExpense	41,604us-gaap_RoyaltyExpense	43,445us-gaap_RoyaltyExpense
Corporate
Operating Leased Assets [Line Items]
Future minimum lease commitments related to contracts of affreightment	$ 58,255

Future minimum lease commitments related to contracts of affreightment.

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- Definition

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41499-112717

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- Definition

Amount of expense related to royalty payments under a contractual arrangement such as payment for mineral and drilling rights and use of technology or intellectual property.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.3)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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