Accumulated other comprehensive loss - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Defined benefit plan adjustment	$ (781)	$ (71)

Other Comprehensive (Income) Loss, Defined Benefit Plan, before Tax, after Reclassification Adjustment, Attributable to Equity Method Investment

No definition available.

Name:	son_OtherComprehensiveIncomeLossDefinedBenefitPlanbeforeTaxafterReclassificationAdjustmentAttributabletoEquityMethodInvestment
Namespace Prefix:	son_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration