Segment Data (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Data [Abstract]
Number of reportable segments | Segment									2
Revenues [Abstract]
Revenues	$ 708,111	$ 541,274	$ 473,328	$ 491,705	$ 435,650	$ 434,708	$ 441,582	$ 458,733	$ 2,214,418	$ 1,770,673	$ 2,147,532
Segment profit (loss)	(58,860)	47,861	43,015	40,998	52,370	51,559	62,474	61,343	73,014	227,746	361,712
Total assets	5,127,427				4,289,895				5,127,427	4,289,895	4,140,558
Depreciation and amortization									202,881	200,917	192,240
Capital expenditures									177,222	231,066	345,475
Other segment disclosures [Abstract]
Interest expense	(7,162)	(5,388)	(4,465)	(4,457)	(4,477)	(4,507)	(4,513)	(4,193)	(21,472)	(17,690)	(18,738)
Impairment of long-lived assets	(105,712)								(105,712)	0	0
Gain (loss) on disposition of assets	(4,288)	(159)	(139)	99	(166)	(122)	94	67	(4,487)	(127)	1,672
Other income (expense)	469	$ (113)	$ (1)	$ (116)	47	$ (120)	$ 179	$ 135	(52)	(291)	(663)
Profit (loss) from other segment									(149,582)	(32,542)	(32,051)
Details of "Other" total assets [Abstract]
Investment in affiliates	1,890				2,622				1,890	2,622
Other [Member]
Revenues [Abstract]
Segment profit (loss)									(149,582)	(32,542)	(32,051)
Total assets	75,243				52,676				75,243	52,676	63,009
Depreciation and amortization									3,596	3,793	3,944
Capital expenditures									6,715	7,728	4,706
Other segment disclosures [Abstract]
General corporate expenses									(18,150)	(14,966)	(14,773)
Interest expense									(21,472)	(17,690)	(18,738)
Impairment of long-lived assets									(105,712)	0	0
Gain (loss) on disposition of assets									(4,487)	(127)	1,672
Other income (expense)									239	241	(212)
Details of "Other" total assets [Abstract]
General corporate assets	73,353				50,054				73,353	50,054	60,919
Investment in affiliates	1,890				2,622				1,890	2,622	2,090
Total other assets	75,243				52,676				75,243	52,676	63,009
Reportable Segments [Member] | Marine Transportation [Member]
Revenues [Abstract]
Revenues									1,324,106	1,471,893	1,663,090
Segment profit (loss)									136,011	257,102	374,842
Total assets	3,485,099				3,613,951				3,485,099	3,613,951	3,444,785
Depreciation and amortization									178,898	184,291	175,798
Capital expenditures									165,421	217,423	311,862
Reportable Segments [Member] | Distribution and Services [Member]
Revenues [Abstract]
Revenues									890,312	298,780	484,442
Segment profit (loss)									86,585	3,186	18,921
Total assets	$ 1,567,085				$ 623,268				1,567,085	623,268	632,764
Depreciation and amortization									20,387	12,833	12,498
Capital expenditures									5,086	5,915	28,907
Intersegment Eliminations [Member]
Revenues [Abstract]
Revenues									20,717	24,672	26,203
Segment profit (loss)									$ 2,072	$ 2,467	$ 2,620