Consolidated Statements of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Statement [Abstract]
Operating Revenue	$ 226,785	$ 133,992	$ 451,265	$ 268,265
Operating Expenses
Salaries, wages and benefits	71,240	40,939	142,185	81,537
Rent and purchased transportation	13,741	1,325	28,252	2,623
Fuel	59,338	38,637	122,563	81,615
Operations and maintenance	9,776	3,828	19,897	9,269
Operating taxes and licenses	5,319	2,468	10,165	4,884
Insurance and claims	4,370	4,744	11,465	7,604
Communications and utilities	1,553	699	3,383	1,472
Depreciation and amortization	26,668	16,930	51,241	31,995
Other operating expenses	7,997	3,691	16,687	7,506
Gain on disposal of property and equipment	(13,859)	(8,644)	(15,903)	(19,823)
Total operating expenses	186,143	104,617	389,935	208,682
Operating income	40,642	29,375	61,330	59,583
Interest income	106	129	142	253
Interest expense	(132)	0	(287)	0
Income before income taxes	40,616	29,504	61,185	59,836
Federal and state income taxes	14,144	10,366	20,634	20,962
Net income	26,472	19,138	40,551	38,874
Other comprehensive income, net of tax	0	600	0	600
Comprehensive income	$ 26,472	$ 19,738	$ 40,551	$ 39,474
Net income per share
Basic	$ 0.30	$ 0.23	$ 0.46	$ 0.46
Diluted	$ 0.30	$ 0.23	$ 0.46	$ 0.46
Weighted average shares outstanding
Basic	87,728	84,789	87,716	84,780
Diluted	87,900	85,039	87,908	85,042
Dividends declared per share	$ 0.02	$ 0.02	$ 0.04	$ 0.04