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Fair Value - Reconciliation of Beginning and Ending Balances for Assets (Liabilities) Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level Three) (Detail) - Fuel price derivatives – diesel [Member] - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2015		Sep. 30, 2014		Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 6,078		$ (1,925)		$ 11,848	$ (2,142)
Included in earnings	(1,633)	[1]	4,211	[1]	(7,403)	4,428
Included in other comprehensive income	0		0		0	0
Ending balance	$ 4,445		$ 2,286		$ 4,445	$ 2,286
[1] Gains and losses (realized and unrealized) associated with fuel price derivatives, included in earnings for the three months ended September 30, 2015 and 2014, are reported in net realized and unrealized gain on fuel price derivative instruments on the unaudited condensed consolidated statements of income.

X

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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



-Publisher FASB



-Name Accounting Standards Codification



-Topic 820



-SubTopic 10



-Section 50



-Paragraph 2



-Subparagraph (c)(1)



-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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



-Publisher FASB



-Name Accounting Standards Codification



-Topic 820



-SubTopic 10



-Section 50



-Paragraph 2



-Subparagraph (c)(1a)



-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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



-Publisher FASB



-Name Accounting Standards Codification



-Topic 820



-SubTopic 10



-Section 50



-Paragraph 2



-Subparagraph (c)(1)



-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258



Reference 2: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Topic 820



-SubTopic 10



-Section 50



-Paragraph 3



-Subparagraph (b)



-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19279-110258

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