Condensed Consolidating Guarantor Financial Information Condensed Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Cash Flow Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities	$ (14,658)	$ 53,500	$ 217,095
Costs Incurred For Offshore Supply Vessels Newbuild Program	(18,104)	(76,277)	(190,070)
Proceeds from Sale of Property, Plant, and Equipment	43	524	152,000
Payments to Acquire Property, Plant, and Equipment	(1,687)	(20,689)	(86,792)
Payments to Acquire Other Property, Plant, and Equipment	(1,552)	(569)	(16,487)
Net Cash Provided by (Used in) Investing Activities	(21,300)	(97,011)
Proceeds from Lines of Credit	66,640	0	0
Repayments of Senior Debt	(5,057)	0	0
Repayments of Convertible Debt	(49,631)	0	0
Payments of Financing Costs	(5,636)	(1,102)	(2,089)
Payments Related to Tax Withholding for Share-based Compensation	(575)	(450)	(1,252)
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options	485	1,300	3,112
Net Cash Provided by (Used in) Financing Activities	6,226	(252)
Effect of Exchange Rate on Cash and Cash Equivalents	(446)	989	(839)
Cash and Cash Equivalents, Period Increase (Decrease)	(30,178)	(42,774)	74,678
Cash and Cash Equivalents, at Carrying Value	186,849	217,027	259,801	$ 185,123
Cash Paid for Interest	52,194	50,152	50,492
Income Taxes Paid		3,732	4,808
Proceeds from Income Tax Refunds	(9,042)
Notes Reduction	127,096	0	0
Consolidation, Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities	0	0
Costs Incurred For Offshore Supply Vessels Newbuild Program	0	0
Proceeds from Sale of Property, Plant, and Equipment	0	0
Payments to Acquire Property, Plant, and Equipment	0	0
Payments to Acquire Other Property, Plant, and Equipment	0	0
Net Cash Provided by (Used in) Investing Activities	0	0
Payments of Financing Costs		0
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options		0
Net Cash Provided by (Used in) Financing Activities		0
Effect of Exchange Rate on Cash and Cash Equivalents		0
Cash and Cash Equivalents, Period Increase (Decrease)		0
Cash and Cash Equivalents, at Carrying Value	0	0	0
Cash Paid for Interest		0
Income Taxes Paid		0
Parent Company [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities	(6,231)	251
Costs Incurred For Offshore Supply Vessels Newbuild Program	0	0
Proceeds from Sale of Property, Plant, and Equipment	0	0
Payments to Acquire Property, Plant, and Equipment	0	0
Payments to Acquire Other Property, Plant, and Equipment	0	0
Net Cash Provided by (Used in) Investing Activities	0	0
Proceeds from Lines of Credit	66,640
Repayments of Senior Debt	(5,057)
Repayments of Convertible Debt	(49,631)
Payments of Financing Costs	5,636	(1,102)
Payments Related to Tax Withholding for Share-based Compensation	(575)
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options	485	1,300
Net Cash Provided by (Used in) Financing Activities	6,226	(252)
Effect of Exchange Rate on Cash and Cash Equivalents	0	0
Cash and Cash Equivalents, Period Increase (Decrease)	(5)	(1)
Cash and Cash Equivalents, at Carrying Value	4	9	10
Cash Paid for Interest	52,194
Income Taxes Paid	0	0
Notes Reduction	127,096
Guarantor Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities	(12,152)	55,677
Costs Incurred For Offshore Supply Vessels Newbuild Program	(18,496)	(76,615)
Proceeds from Sale of Property, Plant, and Equipment	33	523
Payments to Acquire Property, Plant, and Equipment	(1,173)	(19,604)
Payments to Acquire Other Property, Plant, and Equipment	(1,512)	(467)
Net Cash Provided by (Used in) Investing Activities	(21,148)	(96,163)
Proceeds from Lines of Credit	0
Repayments of Senior Debt	0
Repayments of Convertible Debt	0
Payments of Financing Costs	0	0
Payments Related to Tax Withholding for Share-based Compensation	0
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options	0	0
Net Cash Provided by (Used in) Financing Activities	0	0
Effect of Exchange Rate on Cash and Cash Equivalents	(150)	31
Cash and Cash Equivalents, Period Increase (Decrease)	(33,450)	(40,455)
Cash and Cash Equivalents, at Carrying Value	178,746	212,196	252,651
Cash Paid for Interest	0	0
Income Taxes Paid	(9,793)	1,292
Notes Reduction	0
Non-Guarantor Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities	3,725	(2,428)
Costs Incurred For Offshore Supply Vessels Newbuild Program	392	338
Proceeds from Sale of Property, Plant, and Equipment	10	1
Payments to Acquire Property, Plant, and Equipment	(514)	(1,085)
Payments to Acquire Other Property, Plant, and Equipment	(40)	(102)
Net Cash Provided by (Used in) Investing Activities	(152)	(848)
Proceeds from Lines of Credit	0
Repayments of Senior Debt	0
Repayments of Convertible Debt	0
Payments of Financing Costs	0	0
Payments Related to Tax Withholding for Share-based Compensation	0
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options	0	0
Net Cash Provided by (Used in) Financing Activities	0	0
Effect of Exchange Rate on Cash and Cash Equivalents	(296)	958
Cash and Cash Equivalents, Period Increase (Decrease)	3,277	(2,318)
Cash and Cash Equivalents, at Carrying Value	8,099	4,822	$ 7,140
Cash Paid for Interest	0
Income Taxes Paid	751	2,440
Notes Reduction	0
Consolidation, Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Proceeds from Lines of Credit	0
Repayments of Senior Debt
Repayments of Convertible Debt	0
Payments of Financing Costs	0
Payments Related to Tax Withholding for Share-based Compensation	0
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options	0
Net Cash Provided by (Used in) Financing Activities	0
Effect of Exchange Rate on Cash and Cash Equivalents	0
Cash and Cash Equivalents, Period Increase (Decrease)	0
Cash and Cash Equivalents, at Carrying Value	0	$ 0
Cash Paid for Interest	0
Income Taxes Paid	0
Notes Reduction	$ 0