Assets and Liabilities Measured at Fair Value and Related Hierarchy Levels (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Fair Value Measurements [Line Items]
Total available-for-sale securities	$ 16,058		$ 15,963
Executive deferred compensation plan trust	5,315	[1]	4,339	[1]
Total fuel price derivatives - liabilities	4,535		7,358
Mortgage-backed securities
Fair Value Measurements [Line Items]
Total available-for-sale securities	840		839
Asset-backed securities
Fair Value Measurements [Line Items]
Total available-for-sale securities	1,337		1,391
Municipal bonds
Fair Value Measurements [Line Items]
Total available-for-sale securities	522		519
Equity securities
Fair Value Measurements [Line Items]
Total available-for-sale securities	13,359		13,214
Fuel price derivatives - unleaded fuel
Fair Value Measurements [Line Items]
Total fuel price derivatives - liabilities	3,912		5,216	[2]
Fuel price derivatives - diesel
Fair Value Measurements [Line Items]
Total fuel price derivatives - liabilities	623	[2]	2,142	[2]
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Measurements [Line Items]
Total available-for-sale securities	13,359		13,214
Executive deferred compensation plan trust	5,315	[1]	4,339	[1]
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Fair Value Measurements [Line Items]
Total available-for-sale securities	13,359		13,214
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fuel price derivatives - diesel
Fair Value Measurements [Line Items]
Total fuel price derivatives - liabilities	0	[2]
Significant Other Observable Inputs (Level 2)
Fair Value Measurements [Line Items]
Total available-for-sale securities	2,699		2,749
Total fuel price derivatives - liabilities	3,912		5,216
Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Fair Value Measurements [Line Items]
Total available-for-sale securities	840		839
Significant Other Observable Inputs (Level 2) | Asset-backed securities
Fair Value Measurements [Line Items]
Total available-for-sale securities	1,337		1,391
Significant Other Observable Inputs (Level 2) | Municipal bonds
Fair Value Measurements [Line Items]
Total available-for-sale securities	522		519
Significant Other Observable Inputs (Level 2) | Fuel price derivatives - unleaded fuel
Fair Value Measurements [Line Items]
Total fuel price derivatives - liabilities	3,912		5,216	[2]
Significant Other Observable Inputs (Level 2) | Fuel price derivatives - diesel
Fair Value Measurements [Line Items]
Total fuel price derivatives - liabilities	0	[2]
Significant Other Observable Inputs (Level 3)
Fair Value Measurements [Line Items]
Total fuel price derivatives - liabilities	623		2,142
Significant Other Observable Inputs (Level 3) | Fuel price derivatives - diesel
Fair Value Measurements [Line Items]
Total fuel price derivatives - liabilities	$ 623	[2]	$ 2,142	[2]

[1] The fair value of these instruments is recorded in other assets.

[2] The balance sheet presentation combines unleaded fuel and diesel fuel positions.