Fair Value Measurements - Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities held in the deferred compensation plans	$ 61,717,000	$ 62,376,000
Fair Value, Measurements, Recurring | Swaps
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	(207,979,000)	283,276,000
Fair Value, Measurements, Recurring | Swaps | Freight
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	65,000
Fair Value, Measurements, Recurring | Collars
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	3,673,000
Fair Value, Measurements, Recurring | Puts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	18,159,000
Fair Value, Measurements, Recurring | Calls
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	(1,041,000)
Fair Value, Measurements, Recurring | Basis Swaps
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	11,106,000	4,329,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities held in the deferred compensation plans	61,717,000	62,376,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Swaps
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	0	0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Swaps | Freight
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Collars
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Puts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Calls
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Basis Swaps
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	0	0
Fair Value, Inputs, Level 2 | Swaps | Freight
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	65,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities held in the deferred compensation plans	0	0
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Swaps
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	(207,979,000)	283,276,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Swaps | Freight
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	65,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Collars
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	3,673,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Puts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	18,159,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Calls
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	(1,041,000)
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Basis Swaps
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	11,106,000	4,329,000
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities held in the deferred compensation plans	0	0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Swaps
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	0	0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Swaps | Freight
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Collars
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Puts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Calls
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Basis Swaps
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	$ 0	$ 0