Long-Term Debt (Narrative) (Senior Notes And Convertible Debentures) (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Jul. 03, 2017 USD ($)	Nov. 15, 2016 USD ($)	Jun. 01, 2015 USD ($)	Apr. 15, 2015 USD ($)	Nov. 13, 2013 USD ($)	Dec. 11, 2012 USD ($)	Sep. 24, 2009 USD ($)
Debt Instrument [Line Items]
Common Stock, Par or Stated Value Per Share | $ / shares		$ 0.01	$ 0.01
Stock Repurchase Program, Authorized Amount								$ 150,000,000
Convertible Securities | shares		5,258,065	0	0
Debt extinguishment gains (losses), net		$ (819,000)	$ 5,184,000	$ (28,497,000)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt		28,725,000	0	0
Letters of credit outstanding amount	$ 27,200,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount		77,400,000
Sea-Vista [Member]
Debt Instrument [Line Items]
Payments of Debt Issuance Costs				3,100,000
Debt extinguishment gains (losses), net		$ 700,000
Line of Credit Facility, Maximum Borrowing Capacity										$ 300,000,000
Sea-Vista [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Mandatory Prepayment Balance				75,000,000
Debt Instrument, Prepayment Increments				1,000,000
Sea-Vista [Member] | Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		2.00%
Sea-Vista [Member] | Minimum [Member] | Base Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.25%
Sea-Vista [Member] | Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		2.75%
Sea-Vista [Member] | Maximum [Member] | Base Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.75%
ISH [Member]
Debt Instrument [Line Items]
Payments of Debt Issuance Costs		$ 100,000
Line of Credit Facility, Maximum Borrowing Capacity							$ 25,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage		0.50%
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt							28,700,000
ISH [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Mandatory Prepayment Balance		$ 700,000
SEACOR Marine Holdings Inc. [Member]
Debt Instrument [Line Items]
Letters of credit outstanding amount		16,700,000
Guarantor Obligations, Maximum Exposure, Undiscounted		63,800,000
Guaranty Fee Income		$ 600,000	800,000
Three Point Zero Percentage Convertible Notes [Member]
Debt Instrument [Line Items]
Debt instrument, stated percentage		3.00%									3.00%
Aggregate principal amount											$ 230,000,000
Bond Trading Price Triggering Contingent Interest, Minimum					$ 1,200
Individual Bond, Face Value											$ 1,000
Contingent Interest Rate											0.45%
Debt Instrument, Convertible, Conversion Ratio					12.5892
Convertible Securities | shares		2,895,516
Bond Redemption Percentage		100.00%
Convertible Debt		$ 230,000,000	$ 230,000,000
Debt Instrument, Interest Rate, Effective Percentage			7.40%
Two Point Five Percentage Convertible Notes [Member]
Debt Instrument [Line Items]
Debt instrument, stated percentage		2.50%	2.50%									2.50%
Aggregate principal amount												$ 350,000,000.0
Bond Trading Price Triggering Contingent Interest, Minimum						$ 1,200
Individual Bond, Face Value												$ 1,000
Contingent Interest Rate												0.25%
Debt Instrument, Convertible, Conversion Ratio		19.0381
Convertible Securities | shares		1,227,101
Bond Redemption Percentage		100.00%
Convertible Debt		$ 64,455,000	$ 157,128,000
Debt Instrument, Interest Rate, Effective Percentage		6.50%
Debt Instrument, Repurchased Face Amount		$ 61,700,000	127,400,000	65,500,000
Debt Instrument, Repurchase Amount		61,900,000	124,700,000	62,600,000
Debt extinguishment gains (losses), net		100,000	3,300,000	1,100,000
Two Point Five Percentage Convertible Notes [Member] | Additional Paid-In Capital [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchase Amount		1,400,000	7,400,000	3,000,000
7.375% Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, stated percentage													7.375%
Aggregate principal amount													$ 250,000,000
Debt Instrument, Repurchased Face Amount		7,600,000	35,200,000	37,600,000
Debt Instrument, Repurchase Amount		7,700,000	33,100,000	37,900,000
Debt extinguishment gains (losses), net		(200,000)	1,900,000	600,000
Revolving Credit Facility [Member] | Sea-Vista [Member]
Debt Instrument [Line Items]
Debt Instrument, Unused Borrowing Capacity, Amount										100,000,000
Proceeds from Lines of Credit		44,900,000	87,000,000	30,000,000
Repayments of Lines of Credit		85,900,000	14,000,000	17,000,000
Line of Credit Facility, Current Borrowing Capacity		55,000,000
Revolving Credit Facility [Member] | ISH [Member]
Debt Instrument [Line Items]
Debt Instrument, Unused Borrowing Capacity, Amount							5,000,000
Repayments of Long-term Debt	5,000,000
Line of Credit Facility, Current Borrowing Capacity		$ 5,000,000
Other ISH Debt [Member] | ISH [Member]
Debt Instrument [Line Items]
Debt instrument, stated percentage		7.00%
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt							3,900,000
ISH Term Loan [Member] | ISH [Member]
Debt Instrument [Line Items]
Debt Instrument, Unused Borrowing Capacity, Amount							$ 20,000,000
Repayments of Long-term Debt	$ 7,800,000
Term A-1 Loan [Member] | Sea-Vista [Member]
Debt Instrument [Line Items]
Proceeds from issuance of long-term debt, net of offering costs				$ 80,000,000
Debt Instrument, Unused Borrowing Capacity, Amount										80,000,000
Debt Instrument, Periodic Payment, Principal, Percent				1.25%
Repayments of Long-term Debt		$ 39,400,000	3,800,000	$ 3,000,000
Term A-2 Loan [Member] | Sea-Vista [Member]
Debt Instrument [Line Items]
Proceeds from issuance of long-term debt, net of offering costs				$ 120,000,000
Debt Instrument, Unused Borrowing Capacity, Amount										$ 120,000,000
Debt Instrument, Periodic Payment, Principal, Percent				2.50%
Repayments of Long-term Debt		63,100,000
Title XI Bonds [Member]
Debt Instrument [Line Items]
Debt extinguishment gains (losses), net				$ 29,000,000
Repayments of Long-term Debt				99,900,000
Redemption Premium				20,500,000
Restricted Cash and Cash Equivalents									$ 9,600,000
Other Debt Obligations [Member]
Debt Instrument [Line Items]
Proceeds from issuance of long-term debt, net of offering costs			7,400,000	4,900,000
Repayments of Long-term Debt		$ 3,000,000	400,000	9,100,000
Other Debt Obligations [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, stated percentage		3.40%
Other Debt Obligations [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, stated percentage		4.30%
Long-term Debt [Member] | Two Point Five Percentage Convertible Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchase Amount		$ 60,500,000	$ 117,300,000	$ 59,600,000
Put Option [Member] | Two Point Five Percentage Convertible Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount		31,000,000
Debt Instrument, Repurchase Amount		$ 31,000,000