Basis of Presentation and Accounting Policies - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Basis Of Presentation And Accounting Policies [Line Items]
Contract with customer, liability	$ 2,627,000	$ 3,265,000	$ 3,307,000	$ 4,755,000
Contract with customer liability current	4,567,000		4,452,000
Construction reserve funds, current	4,200,000		$ 4,300,000
Capitalized interest costs, including allowance for funds used during construction, total	200,000	200,000
Impairment of long-lived assets owned or leased in vessels	0
Impairment of long-lived assets one owned liftboat		1,800,000
Impairment of long-lived assets two leased-in liftboat		7,400,000
Equity method investment, other than temporary impairment	$ 0	$ 0
Effective income tax rate reconciliation, percent, total	345.20%	27.10%
Class of warrant or right, exercise price of warrants or rights	$ 0.01
Incremental common shares attributable to conversion of debt securities, total	2,183,708	2,183,708
Restricted Stock
Basis Of Presentation And Accounting Policies [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	1,059,077	461,363
Employee Stock Option
Basis Of Presentation And Accounting Policies [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount	1,120,541	921,069
Offshore Support Vessels
Basis Of Presentation And Accounting Policies [Line Items]
Estimated useful life (Year)	20 years
Prepaid Vessel Management Fees
Basis Of Presentation And Accounting Policies [Line Items]
Contract with customer liability current	$ 1,500,000
Prepaid Charter Modification and Reservation Fees
Basis Of Presentation And Accounting Policies [Line Items]
Contract with customer liability current	400,000
Geographic Distribution, Domestic
Basis Of Presentation And Accounting Policies [Line Items]
Contract with customer, liability	2,600,000
Geographic Distribution, Foreign
Basis Of Presentation And Accounting Policies [Line Items]
Contract with customer, liability	$ 700,000
Minimum
Basis Of Presentation And Accounting Policies [Line Items]
Equity method investment, ownership percentage	20.00%
Minimum | SEACOR Marine Foreign Holdings
Basis Of Presentation And Accounting Policies [Line Items]
Equity method investment, ownership percentage	50.00%
Maximum
Basis Of Presentation And Accounting Policies [Line Items]
Equity method investment, ownership percentage	50.00%	50.00%
Accounting Standards Update 2019-12
Basis Of Presentation And Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, immaterial effect [true false]	true