Accumulated other comprehensive loss - Accumulated Other Comprehensive Income Loss and Changes in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 1,772,278	$ 1,730,060
Ending Balance	1,815,705	1,772,278
Foreign Currency Items
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(251,102)	(198,495)
Other comprehensive income/(loss) before reclassifications	9,108	(53,504)
Other comprehensive income/(loss)	9,108	(52,607)
Ending Balance	(241,994)	(251,102)
Foreign Currency Items \| Fixed assets
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss	0	0
Foreign Currency Items \| Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss		0
Foreign Currency Items \| Net Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss	0	897
Defined Benefit Pension Items
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(487,380)	(467,136)
Other comprehensive income/(loss) before reclassifications	(111,493)	(50,232)
Other comprehensive income/(loss)	(87,033)	(20,244)
Ending Balance	(574,413)	(487,380)
Defined Benefit Pension Items \| Fixed assets
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss	0	0
Defined Benefit Pension Items \| Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss		0
Defined Benefit Pension Items \| Net Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss	24,460	29,988
Gains and Losses on Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(2,431)	(641)
Other comprehensive income/(loss) before reclassifications	2,061	(1,380)
Other comprehensive income/(loss)	2,035	(1,614)
Ending Balance	(396)	(2,431)
Gains and Losses on Cash Flow Hedges \| Fixed assets
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss	(107)	(305)
Gains and Losses on Cash Flow Hedges \| Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss		(176)
Gains and Losses on Cash Flow Hedges \| Net Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss	81	71
Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(740,913)	(666,272)
Other comprehensive income/(loss) before reclassifications	(100,324)	(105,116)
Amounts reclassified from accumulated other comprehensive loss	(107)	88
Other comprehensive income/(loss)	(75,890)	(74,465)
Ending Balance	(816,803)	(740,913)
Accumulated Other Comprehensive Loss \| Fixed assets
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss	(107)	(305)
Accumulated Other Comprehensive Loss \| Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss		(176)
Accumulated Other Comprehensive Loss \| Net Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Amounts reclassified from accumulated other comprehensive loss	$ 24,541	$ 30,956

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

Name:	us-gaap_IncreaseDecreaseInStockholdersEquityRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount after tax, before reclassification adjustments of other comprehensive income (loss).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669686-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=118951672&loc=d3e1436-108581

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount after tax and reclassification adjustments of other comprehensive income (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669619-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=118951672&loc=d3e1436-108581

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669625-108580

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount after tax of reclassification adjustments of other comprehensive income (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 17B
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL34724394-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669686-108580

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=118951672&loc=d3e1436-108581

+ Details

Name:	us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of stockholders' equity (deficit), net of receivables from officers, directors, owners, and affiliates of the entity, attributable to both the parent and noncontrolling interests. Amount excludes temporary equity. Alternate caption for the concept is permanent equity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4568740-111683

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 55
-Paragraph 4I
-URI http://asc.fasb.org/extlink&oid=116874947&loc=SL4590271-111686

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4568447-111683

+ Details

Name:	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedTranslationAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_PropertyPlantAndEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_RetainedEarningsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=son_NetIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedOtherComprehensiveIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: