Equipment Acquisitions, Dispositions and Depreciation and Impairment Policies (Narrative) (Details) $ in Thousands	3 Months Ended	6 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($)	Jun. 30, 2016 USD ($) equipment	Jun. 30, 2015 USD ($)	Dec. 31, 2015 equipment
Property, Plant and Equipment [Line Items]
Payments to Acquire Property, Plant, and Equipment | $	$ 204,100	$ 204,074	$ 132,128
Sales Price Of Equipment | $		153,900
Proceeds from disposition of property and equipment | $		145,159	22,686
Proceeds from Issuance of Secured Debt | $		8,000
Gain (Loss) on Disposition of Property Plant Equipment | $		11,500
Increase (Decrease) in Deferred Charges | $		9,003	2,035
Amortization Of Deferred Gains Included In Gains On Asset Dispositions And Impairments, Net | $		2,850	0
Sale Leaseback Transaction, Net Proceeds, Investing Activities | $		$ 61,000
Sale Leaseback Transaction, Lease Terms		76
Offshore Support Vessels Standby Safety [Member]
Property, Plant and Equipment [Line Items]
Number Of Equipments Sold | equipment		2
Offshore Support Vessels Fast Support [Member]
Property, Plant and Equipment [Line Items]
Number of Equipment Acquired | equipment		1
Offshore Support Vessels Supply [Member]
Property, Plant and Equipment [Line Items]
Number Of Equipments Delivered | equipment				1
Wind Farm Utility Vessel [Member]
Property, Plant and Equipment [Line Items]
Number Of Equipments Delivered | equipment				1
Inland River Towboat [Member]
Property, Plant and Equipment [Line Items]
Number Of Equipments Delivered | equipment				1
Number Of Equipments Sold | equipment		14
U.S.-flag Product Tankers [Member]
Property, Plant and Equipment [Line Items]
Number Of Equipments Delivered | equipment		2
Sale Leaseback Transaction, Description of Asset(s)		1
Number Of Equipments Sold | equipment		1
U.S.-flag Harbor Tugs [Member]
Property, Plant and Equipment [Line Items]
Number of Equipment Acquired | equipment		1
Noncash or Part Noncash Divestiture, Amount of Consideration Received | $		$ 2,000
Number Of Equipments Sold | equipment		1
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Property, Plant and Equipment [Line Items]
Proceeds from disposition of property and equipment | $		$ 143,900
Inland River Liquid Tank Barges - 30,000 Barrel [Member]
Property, Plant and Equipment [Line Items]
Number Of Equipments Sold | equipment		20
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Property, Plant and Equipment [Line Items]
Proceeds from disposition of property and equipment | $		$ 1,200
Gain (Loss) on Asset Dispositions and Impairments, Net [Member]
Property, Plant and Equipment [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property | $		2,500
Amortization Of Deferred Gains Included In Gains On Asset Dispositions And Impairments, Net | $		$ 1,246	$ 4,597
Property Subject to Operating Lease [Member] | Inland River Liquid Tank Barges - 30,000 Barrel [Member]
Property, Plant and Equipment [Line Items]
Number of Equipments Removed from Service | equipment		8
Construction in Progress [Member] | Inland River Towboat [Member]
Property, Plant and Equipment [Line Items]
Number of Equipments Removed from Service | equipment		1