Unaudited Condensed Consolidated Statements of Comprehensive Income (Loss) (Parenthetical)	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Statement Of Income And Comprehensive Income [Abstract]
Reclassification of derivative losses on cash flow hedges to equity in earnings	50.00%	50.00%

Percentage of owned companies derivative losses on cash flow hedges reclassification to equity in earnings.

No definition available.

Name:	smhi_PercentageOfOwnedCompaniesDerivativeLossesOnCashFlowHedgesReclassificationToEquityInEarnings
Namespace Prefix:	smhi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration