Segment Data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Data [Abstract]
Number of Reportable Segments									2
Revenues [Abstract]
Marine transportation									$ 1,408,893	$ 1,194,607	$ 915,046
Diesel engine services									703,765	655,810	194,511
Total revenues	512,551	521,324	511,848	566,935	550,145	563,582	437,331	299,359	2,112,658	1,850,417	1,109,557
Segment profit (loss)	92,805	86,525	77,827	83,369	87,279	86,328	68,279	52,861	340,526	294,747	189,640
Total assets	3,653,128				2,960,411				3,653,128	2,960,411	1,794,937
Depreciation and amortization									145,147	126,029	95,296
Capital expenditures									312,167	226,238	136,841
Other segment disclosures [Abstract]
Loss on disposition of assets	15	40	(69)	28	111	(97)	(40)	66	14	40	78
Interest expense	(6,588)	(6,056)	(5,901)	(5,840)	(5,817)	(5,974)	(3,278)	(2,833)	(24,385)	(17,902)	(10,960)
Other income (expense)	(45)	(56)	30	149	183	(6)	78	51	(198)	(41)	273
Details of "Other" total assets [Abstract]
Investment in affiliates	1,808				3,682				1,808	3,682
Total assets	3,653,128				2,960,411				3,653,128	2,960,411	1,794,937
Marine transportation [Member]
Revenues [Abstract]
Segment profit (loss)									311,755	262,193	192,758
Total assets	2,951,723				2,307,821				2,951,723	2,307,821	1,383,252
Depreciation and amortization									129,857	111,292	88,710
Capital expenditures									274,351	218,777	132,744
Details of "Other" total assets [Abstract]
Total assets	2,951,723				2,307,821				2,951,723	2,307,821	1,383,252
Diesel engine services [Member]
Revenues [Abstract]
Segment profit (loss)									66,386	68,105	20,553
Total assets	647,986				608,886				647,986	608,886	185,824
Depreciation and amortization									12,030	11,801	4,055
Capital expenditures									20,477	5,576	920
Details of "Other" total assets [Abstract]
Total assets	647,986				608,886				647,986	608,886	185,824
Other [Member]
Revenues [Abstract]
Segment profit (loss)									(37,615)	(35,551)	(23,671)
Total assets	53,419				43,704				53,419	43,704	225,861
Depreciation and amortization									3,260	2,936	2,531
Capital expenditures									17,339	1,885	3,177
Other segment disclosures [Abstract]
General corporate expenses									(13,294)	(17,915)	(13,189)
Loss on disposition of assets									(14)	(40)	(78)
Interest expense									(24,385)	(17,902)	(10,960)
Other income (expense)									78	306	556
Profit (loss) from other segment									(37,615)	(35,551)	(23,671)
Details of "Other" total assets [Abstract]
General corporate assets	51,611				40,022				51,611	40,022	222,525
Investment in affiliates	1,808				3,682				1,808	3,682	3,336
Total assets	$ 53,419				$ 43,704				$ 53,419	$ 43,704	$ 225,861