Class of warrant or right warrant term.

No definition available.

Name:	ssw_ClassOfWarrantOrRightWarrantTerm
Namespace Prefix:	ssw_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Class of warrant or rights exercised.

+ References

No definition available.

+ Details

Name:	ssw_ClassOfWarrantOrRightsExercised
Namespace Prefix:	ssw_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of director rights to designated by investors.

+ References

No definition available.

+ Details

Name:	ssw_NumberOfDirectorRightsToDesignatedByInvestors
Namespace Prefix:	ssw_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Termination fee paid shares.

+ References

No definition available.

+ Details

Name:	ssw_TerminationFeePaidShares
Namespace Prefix:	ssw_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Termination fee paid value.

+ References

No definition available.

+ Details

Name:	ssw_TerminationFeePaidValue
Namespace Prefix:	ssw_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount for accounts payable to related parties. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39603-107864

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39622-107864

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(k)(1))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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

- Definition

Amount of receivables arising from transactions with related parties due within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39603-107864

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(k)(1))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39622-107864

+ Details

Name:	us-gaap_AccountsReceivableRelatedPartiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of voting equity interests acquired at the acquisition date in the business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

+ Details

Name:	us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of equity in the acquiree held by the acquirer immediately before the acquisition date in a business combination.

+ References

No definition available.

+ Details

Name:	us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Exercise price per share or per unit of warrants or rights outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(4))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of warrants or rights outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ClassOfWarrantOrRightOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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

- Definition

The interest rate applicable to the portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a set, unchanging rate.

+ References

+ Details

Name:	us-gaap_LongTermDebtPercentageBearingFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash inflow associated with the amount received from holders exercising their stock warrants.

+ References

No definition available.

+ Details

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

- Definition

Yield on the receivable, on which interest has been imputed, as calculated from its issuance value or purchase price. The calculated effective interest rate considers factors such as the issued face value or price paid for the receivable, the time period between payments, and the time until maturity [full receipt] of the receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

Name:	us-gaap_ReceivableWithImputedInterestEffectiveYieldInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Receivables to be collected from (obligations owed to) related parties, net as of the balance sheet date within one year where one party can exercise control or significant influence over another party; including affiliates, owners or officers and their immediate families, pension trusts, and so forth.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(k)(1))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_RelatedPartyTransactionDueFromToRelatedPartyCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TitleOfIndividualAxis=us-gaap_DirectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=ssw_GreaterChinaInternationalInvestmentsLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=ssw_VesselsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=ssw_OtherRelatedPartiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=ssw_FairfaxFinancialHoldingsLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=ssw_TwoZeroOneNineWarrantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=ssw_SeniorNotesDueTwoZeroTwoFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=ssw_SeniorNotesDueTwoZeroTwoSixMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related party transactions - Additional Information (Detail)					12 Months Ended
	Jul. 16, 2018 $ / shares shares	May 31, 2018 USD ($)	Mar. 13, 2018 USD ($) $ / shares shares	Feb. 14, 2018 USD ($) Director shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Mar. 31, 2018 shares	Mar. 12, 2018
Related Party Transaction [Line Items]
Loans to affiliates					$ 0	$ 36,100,000
Amounts payable to related parties included in accounts payable and accrued liabilities					0	1,386,000
Gross proceeds from exercise of warrants					$ 250,000,000	0	$ 0
Class A Common Shares [Member] \| Director [Member]
Related Party Transaction [Line Items]
Termination fee paid						$ 6,250,000
Termination fee paid, shares \| shares						945,537
GCI [Member]
Related Party Transaction [Line Items]
Percentage of equity method investment									10.80%
Percentage of interest in acquired entity			89.20%
Amounts payable to related parties included in accounts payable and accrued liabilities						$ 1,385,500
GCI [Member] \| Vessels [Member]
Related Party Transaction [Line Items]
Loans to affiliates						36,100,000
GCI [Member] \| Vessels [Member] \| Minimum [Member]
Related Party Transaction [Line Items]
Receivable interest rate per annum					5.00%
GCI [Member] \| Vessels [Member] \| Maximum [Member]
Related Party Transaction [Line Items]
Receivable interest rate per annum					6.00%
Other Related Parties [Member]
Related Party Transaction [Line Items]
Amounts due from related other parties included in accounts receivable					$ 255,000	$ 318,500
Fairfax Financial Holdings Limited [Member]
Related Party Transaction [Line Items]
Warrants outstanding \| shares				38,461,539
Gross proceeds from exercise of warrants		$ 500,000,000
Fairfax Financial Holdings Limited [Member] \| Class A Common Shares [Member]
Related Party Transaction [Line Items]
Warrants outstanding \| shares	25,000,000
Warrants outstanding exercise price \| $ / shares	$ 8.05
Warrants outstanding term	7 years
Fairfax Financial Holdings Limited [Member] \| 2019 Warrants [Member] \| Class A Common Shares [Member]
Related Party Transaction [Line Items]
Warrants outstanding \| shares								38,461,539
Warrants outstanding exercise price \| $ / shares			$ 6.50
Warrants outstanding exercised \| shares			38,461,539
Fairfax Financial Holdings Limited [Member] \| Senior Notes Due 2025 [Member]
Related Party Transaction [Line Items]
Face value of debt				$ 250,000,000
Interest bearing of aggregate principal amount				5.50%
Number of director rights to designated by investors \| Director				2
Fairfax Financial Holdings Limited [Member] \| Senior Notes Due 2026 [Member]
Related Party Transaction [Line Items]
Face value of debt			$ 250,000,000
Interest bearing of aggregate principal amount			5.50%